diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index 98f09fe378c..56e67fe2143 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -50,6 +50,12 @@
 		"./vs/base/browser/ui/scrollbar/verticalScrollbar.ts",
 		"./vs/base/browser/ui/splitview/panelview.ts",
 		"./vs/base/browser/ui/splitview/splitview.ts",
+		"./vs/base/browser/ui/tree/abstractTree.ts",
+		// "./vs/base/browser/ui/tree/dataTree.ts",
+		// "./vs/base/browser/ui/tree/indexTree.ts",
+		// "./vs/base/browser/ui/tree/indexTreeModel.ts",
+		// "./vs/base/browser/ui/tree/objectTree.ts",
+		// "./vs/base/browser/ui/tree/objectTreeModel.ts",
 		"./vs/base/browser/ui/tree/tree.ts",
 		"./vs/base/browser/ui/widget.ts",
 		"./vs/base/common/json.ts",
diff --git a/src/vs/base/browser/ui/list/list.ts b/src/vs/base/browser/ui/list/list.ts
index b9815b6ef0b..cb0be292bff 100644
--- a/src/vs/base/browser/ui/list/list.ts
+++ b/src/vs/base/browser/ui/list/list.ts
@@ -44,7 +44,7 @@ export interface IListGestureEvent<T> {
 
 export interface IListContextMenuEvent<T> {
 	browserEvent: UIEvent;
-	element: T;
+	element: T | undefined;
 	index: number;
-	anchor: HTMLElement | { x: number; y: number; };
+	anchor: HTMLElement | { x: number; y: number; } | undefined;
 }
diff --git a/src/vs/base/browser/ui/list/listView.ts b/src/vs/base/browser/ui/list/listView.ts
index a893aa7d305..a045bf269a8 100644
--- a/src/vs/base/browser/ui/list/listView.ts
+++ b/src/vs/base/browser/ui/list/listView.ts
@@ -3,7 +3,7 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 
-import { getOrDefault } from 'vs/base/common/objects';
+import { getOrDefault2 } from 'vs/base/common/objects';
 import { IDisposable, dispose } from 'vs/base/common/lifecycle';
 import { Gesture, EventType as TouchEventType, GestureEvent } from 'vs/base/browser/touch';
 import * as DOM from 'vs/base/browser/dom';
@@ -39,7 +39,7 @@ interface IItem<T> {
 	element: T;
 	size: number;
 	templateId: string;
-	row: IRow;
+	row: IRow | null;
 }
 
 export interface IListViewOptions {
@@ -48,7 +48,7 @@ export interface IListViewOptions {
 	setRowLineHeight?: boolean;
 }
 
-const DefaultOptions: IListViewOptions = {
+const DefaultOptions = {
 	useShadows: true,
 	verticalScrollMode: ScrollbarVisibility.Auto,
 	setRowLineHeight: true
@@ -105,8 +105,8 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 		this.scrollableElement = new ScrollableElement(this.rowsContainer, {
 			alwaysConsumeMouseWheel: true,
 			horizontal: ScrollbarVisibility.Hidden,
-			vertical: getOrDefault(options, o => o.verticalScrollMode, DefaultOptions.verticalScrollMode),
-			useShadows: getOrDefault(options, o => o.useShadows, DefaultOptions.useShadows)
+			vertical: getOrDefault2(options, o => o.verticalScrollMode, DefaultOptions.verticalScrollMode),
+			useShadows: getOrDefault2(options, o => o.useShadows, DefaultOptions.useShadows)
 		});
 
 		this._domNode.appendChild(this.scrollableElement.getDomNode());
@@ -125,7 +125,7 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 		const onDragOver = mapEvent(domEvent(this.rowsContainer, 'dragover'), e => new DragMouseEvent(e));
 		onDragOver(this.onDragOver, this, this.disposables);
 
-		this.setRowLineHeight = getOrDefault(options, o => o.setRowLineHeight, DefaultOptions.setRowLineHeight);
+		this.setRowLineHeight = getOrDefault2(options, o => o.setRowLineHeight, DefaultOptions.setRowLineHeight);
 
 		this.layout();
 	}
@@ -242,7 +242,7 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 		return this.items[index].element;
 	}
 
-	domElement(index: number): HTMLElement {
+	domElement(index: number): HTMLElement | null {
 		const row = this.items[index].row;
 		return row && row.domNode;
 	}
@@ -312,18 +312,18 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 			item.row = this.cache.alloc(item.templateId);
 		}
 
-		if (!item.row.domNode.parentElement) {
+		if (!item.row.domNode!.parentElement) {
 			if (beforeElement) {
-				this.rowsContainer.insertBefore(item.row.domNode, beforeElement);
+				this.rowsContainer.insertBefore(item.row.domNode!, beforeElement);
 			} else {
-				this.rowsContainer.appendChild(item.row.domNode);
+				this.rowsContainer.appendChild(item.row.domNode!);
 			}
 		}
 
-		item.row.domNode.style.height = `${item.size}px`;
+		item.row.domNode!.style.height = `${item.size}px`;
 
 		if (this.setRowLineHeight) {
-			item.row.domNode.style.lineHeight = `${item.size}px`;
+			item.row.domNode!.style.lineHeight = `${item.size}px`;
 		}
 
 		this.updateItemInDOM(item, index);
@@ -333,11 +333,11 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 	}
 
 	private updateItemInDOM(item: IItem<T>, index: number): void {
-		item.row.domNode.style.top = `${this.elementTop(index)}px`;
-		item.row.domNode.setAttribute('data-index', `${index}`);
-		item.row.domNode.setAttribute('data-last-element', index === this.length - 1 ? 'true' : 'false');
-		item.row.domNode.setAttribute('aria-setsize', `${this.length}`);
-		item.row.domNode.setAttribute('aria-posinset', `${index + 1}`);
+		item.row!.domNode!.style.top = `${this.elementTop(index)}px`;
+		item.row!.domNode!.setAttribute('data-index', `${index}`);
+		item.row!.domNode!.setAttribute('data-last-element', index === this.length - 1 ? 'true' : 'false');
+		item.row!.domNode!.setAttribute('aria-setsize', `${this.length}`);
+		item.row!.domNode!.setAttribute('aria-posinset', `${index + 1}`);
 	}
 
 	private removeItemFromDOM(index: number): void {
@@ -345,10 +345,10 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 		const renderer = this.renderers.get(item.templateId);
 
 		if (renderer.disposeElement) {
-			renderer.disposeElement(item.element, index, item.row.templateData);
+			renderer.disposeElement(item.element, index, item.row!.templateData);
 		}
 
-		this.cache.release(item.row);
+		this.cache.release(item.row!);
 		item.row = null;
 	}
 
@@ -388,21 +388,21 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 	@memoize get onTap(): Event<IListGestureEvent<T>> { return filterEvent(mapEvent(domEvent(this.rowsContainer, TouchEventType.Tap), e => this.toGestureEvent(e)), e => e.index >= 0); }
 
 	private toMouseEvent(browserEvent: MouseEvent): IListMouseEvent<T> {
-		const index = this.getItemIndexFromEventTarget(browserEvent.target);
+		const index = this.getItemIndexFromEventTarget(browserEvent.target || null);
 		const item = index < 0 ? undefined : this.items[index];
 		const element = item && item.element;
 		return { browserEvent, index, element };
 	}
 
 	private toTouchEvent(browserEvent: TouchEvent): IListTouchEvent<T> {
-		const index = this.getItemIndexFromEventTarget(browserEvent.target);
+		const index = this.getItemIndexFromEventTarget(browserEvent.target || null);
 		const item = index < 0 ? undefined : this.items[index];
 		const element = item && item.element;
 		return { browserEvent, index, element };
 	}
 
 	private toGestureEvent(browserEvent: GestureEvent): IListGestureEvent<T> {
-		const index = this.getItemIndexFromEventTarget(browserEvent.initialTarget);
+		const index = this.getItemIndexFromEventTarget(browserEvent.initialTarget || null);
 		const item = index < 0 ? undefined : this.items[index];
 		const element = item && item.element;
 		return { browserEvent, index, element };
@@ -455,7 +455,7 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 
 			this.dragAndDropScrollTimeout = window.setTimeout(() => {
 				this.cancelDragAndDropScrollInterval();
-				this.dragAndDropScrollTimeout = null;
+				this.dragAndDropScrollTimeout = -1;
 			}, 1000);
 		}
 	}
@@ -463,7 +463,7 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 	private cancelDragAndDropScrollInterval(): void {
 		if (this.dragAndDropScrollInterval) {
 			window.clearInterval(this.dragAndDropScrollInterval);
-			this.dragAndDropScrollInterval = null;
+			this.dragAndDropScrollInterval = -1;
 		}
 
 		this.cancelDragAndDropScrollTimeout();
@@ -472,15 +472,16 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 	private cancelDragAndDropScrollTimeout(): void {
 		if (this.dragAndDropScrollTimeout) {
 			window.clearTimeout(this.dragAndDropScrollTimeout);
-			this.dragAndDropScrollTimeout = null;
+			this.dragAndDropScrollTimeout = -1;
 		}
 	}
 
 	// Util
 
-	private getItemIndexFromEventTarget(target: EventTarget): number {
-		while (target instanceof HTMLElement && target !== this.rowsContainer) {
-			const element = target as HTMLElement;
+	private getItemIndexFromEventTarget(target: EventTarget | null): number {
+		let element: HTMLElement | null = target as (HTMLElement | null);
+
+		while (element instanceof HTMLElement && element !== this.rowsContainer) {
 			const rawIndex = element.getAttribute('data-index');
 
 			if (rawIndex) {
@@ -491,7 +492,7 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 				}
 			}
 
-			target = element.parentElement;
+			element = element.parentElement;
 		}
 
 		return -1;
@@ -532,16 +533,14 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 				if (item.row) {
 					const renderer = this.renderers.get(item.row.templateId);
 					renderer.disposeTemplate(item.row.templateData);
-					item.row = null;
 				}
 			}
 
-			this.items = null;
+			this.items = [];
 		}
 
-		if (this._domNode && this._domNode.parentElement) {
+		if (this._domNode && this._domNode.parentNode) {
 			this._domNode.parentNode.removeChild(this._domNode);
-			this._domNode = null;
 		}
 
 		this.disposables = dispose(this.disposables);
diff --git a/src/vs/base/browser/ui/list/listWidget.ts b/src/vs/base/browser/ui/list/listWidget.ts
index 32d343568b0..84700441bdb 100644
--- a/src/vs/base/browser/ui/list/listWidget.ts
+++ b/src/vs/base/browser/ui/list/listWidget.ts
@@ -180,7 +180,6 @@ class Trait<T> implements ISpliceable<boolean>, IDisposable {
 	}
 
 	dispose() {
-		this.indexes = null;
 		this._onChange = dispose(this._onChange);
 	}
 }
@@ -224,8 +223,9 @@ class TraitSpliceable<T> implements ISpliceable<T> {
 			return this.trait.splice(start, deleteCount, elements.map(e => false));
 		}
 
-		const pastElementsWithTrait = this.trait.get().map(i => this.getId(this.view.element(i)));
-		const elementsWithTrait = elements.map(e => pastElementsWithTrait.indexOf(this.getId(e)) > -1);
+		const getId = this.getId;
+		const pastElementsWithTrait = this.trait.get().map(i => getId(this.view.element(i)));
+		const elementsWithTrait = elements.map(e => pastElementsWithTrait.indexOf(getId(e)) > -1);
 
 		this.trait.splice(start, deleteCount, elementsWithTrait);
 	}
@@ -357,6 +357,11 @@ class DOMFocusController<T> implements IDisposable {
 		}
 
 		const focusedDomElement = this.view.domElement(focus[0]);
+
+		if (!focusedDomElement) {
+			return;
+		}
+
 		const tabIndexElement = focusedDomElement.querySelector('[tabIndex]');
 
 		if (!tabIndexElement || !(tabIndexElement instanceof HTMLElement) || tabIndexElement.tabIndex === -1) {
@@ -421,7 +426,7 @@ class MouseController<T> implements IDisposable {
 			.map(event => {
 				const index = this.list.getFocus()[0];
 				const element = this.view.element(index);
-				const anchor = this.view.domElement(index);
+				const anchor = this.view.domElement(index) || undefined;
 				return { index, element, anchor, browserEvent: event.browserEvent };
 			})
 			.event;
@@ -436,7 +441,7 @@ class MouseController<T> implements IDisposable {
 			.map(browserEvent => {
 				const index = this.list.getFocus()[0];
 				const element = this.view.element(index);
-				const anchor = this.view.domElement(index);
+				const anchor = this.view.domElement(index) || undefined;
 				return { index, element, anchor, browserEvent };
 			})
 			.filter(({ anchor }) => !!anchor)
@@ -974,10 +979,7 @@ export class List<T> implements ISpliceable<T>, IDisposable {
 
 		this.styleElement = DOM.createStyleSheet(this.view.domNode);
 
-		this.styleController = options.styleController;
-		if (!this.styleController) {
-			this.styleController = new DefaultStyleController(this.styleElement, this.idPrefix);
-		}
+		this.styleController = options.styleController || new DefaultStyleController(this.styleElement, this.idPrefix);
 
 		this.spliceable = new CombinedSpliceable([
 			new TraitSpliceable(this.focus, this.view, options.identityProvider),
@@ -987,8 +989,8 @@ export class List<T> implements ISpliceable<T>, IDisposable {
 
 		this.disposables = [this.focus, this.selection, this.view, this._onDidDispose];
 
-		this.onDidFocus = mapEvent(domEvent(this.view.domNode, 'focus', true), () => null);
-		this.onDidBlur = mapEvent(domEvent(this.view.domNode, 'blur', true), () => null);
+		this.onDidFocus = mapEvent(domEvent(this.view.domNode, 'focus', true), () => null!);
+		this.onDidBlur = mapEvent(domEvent(this.view.domNode, 'blur', true), () => null!);
 
 		this.disposables.push(new DOMFocusController(this, this.view));
 
diff --git a/src/vs/base/browser/ui/tree/abstractTree.ts b/src/vs/base/browser/ui/tree/abstractTree.ts
index 904702b3e62..2b339b430b2 100644
--- a/src/vs/base/browser/ui/tree/abstractTree.ts
+++ b/src/vs/base/browser/ui/tree/abstractTree.ts
@@ -14,7 +14,7 @@ import { KeyCode } from 'vs/base/common/keyCodes';
 import { ITreeModel, ITreeNode, ITreeRenderer, ITreeModelOptions } from 'vs/base/browser/ui/tree/tree';
 import { ISpliceable } from 'vs/base/common/sequence';
 
-export function createComposedTreeListOptions<T, R extends { element: T }>(options?: IListOptions<T>): IListOptions<R> {
+export function createComposedTreeListOptions<T, R extends { element: T }>(options?: IListOptions<T>): IListOptions<R> | undefined {
 	if (!options) {
 		return undefined;
 	}
@@ -22,18 +22,20 @@ export function createComposedTreeListOptions<T, R extends { element: T }>(optio
 	let identityProvider: IIdentityProvider<R> | undefined = undefined;
 
 	if (options.identityProvider) {
-		identityProvider = el => options.identityProvider(el.element);
+		const ip = options.identityProvider;
+		identityProvider = el => ip(el.element);
 	}
 
 	let multipleSelectionController: IMultipleSelectionController<R> | undefined = undefined;
 
 	if (options.multipleSelectionController) {
+		const msc = options.multipleSelectionController;
 		multipleSelectionController = {
 			isSelectionSingleChangeEvent(e) {
-				return options.multipleSelectionController.isSelectionSingleChangeEvent({ ...e, element: e.element } as any);
+				return msc.isSelectionSingleChangeEvent({ ...e, element: e.element } as any);
 			},
 			isSelectionRangeChangeEvent(e) {
-				return options.multipleSelectionController.isSelectionRangeChangeEvent({ ...e, element: e.element } as any);
+				return msc.isSelectionRangeChangeEvent({ ...e, element: e.element } as any);
 			}
 		};
 	}
@@ -41,9 +43,10 @@ export function createComposedTreeListOptions<T, R extends { element: T }>(optio
 	let accessibilityProvider: IAccessibilityProvider<R> | undefined = undefined;
 
 	if (options.accessibilityProvider) {
+		const ap = options.accessibilityProvider;
 		accessibilityProvider = {
 			getAriaLabel(e) {
-				return options.accessibilityProvider.getAriaLabel(e.element);
+				return ap.getAriaLabel(e.element);
 			}
 		};
 	}
@@ -192,7 +195,7 @@ export abstract class AbstractTree<T, TFilterData, TRef> implements IDisposable
 		container: HTMLElement,
 		delegate: IListVirtualDelegate<T>,
 		renderers: ITreeRenderer<T, TFilterData, any>[],
-		options?: ITreeOptions<T, TFilterData>
+		options: ITreeOptions<T, TFilterData> = {}
 	) {
 		const treeDelegate = new ComposedTreeDelegate<T, ITreeNode<T, TFilterData>>(delegate);
 
@@ -356,8 +359,12 @@ export abstract class AbstractTree<T, TFilterData, TRef> implements IDisposable
 
 	private onMouseClick(e: IListMouseEvent<ITreeNode<T, TFilterData>>): void {
 		const node = e.element;
-		const location = this.model.getNodeLocation(node);
 
+		if (!node) {
+			return;
+		}
+
+		const location = this.model.getNodeLocation(node);
 		this.model.toggleCollapsed(location);
 	}
 
@@ -436,7 +443,5 @@ export abstract class AbstractTree<T, TFilterData, TRef> implements IDisposable
 	dispose(): void {
 		this.disposables = dispose(this.disposables);
 		this.view.dispose();
-		this.view = null;
-		this.model = null;
 	}
 }
\ No newline at end of file
diff --git a/src/vs/base/browser/ui/tree/tree.ts b/src/vs/base/browser/ui/tree/tree.ts
index f3fd7701877..2d0705b9ccb 100644
--- a/src/vs/base/browser/ui/tree/tree.ts
+++ b/src/vs/base/browser/ui/tree/tree.ts
@@ -94,9 +94,9 @@ export interface ITreeModel<T, TFilterData, TRef> {
 	getNodeLocation(node: ITreeNode<T, any>): TRef;
 	getParentNodeLocation(location: TRef): TRef | null;
 
-	getParentElement(location: TRef): T | null;
-	getFirstChildElement(location: TRef): T | null;
-	getLastAncestorElement(location: TRef): T | null;
+	getParentElement(location: TRef | null): T | null;
+	getFirstChildElement(location: TRef | null): T | null;
+	getLastAncestorElement(location: TRef | null): T | null;
 
 	isCollapsed(location: TRef): boolean;
 	setCollapsed(location: TRef, collapsed: boolean): boolean;
diff --git a/src/vs/base/common/objects.ts b/src/vs/base/common/objects.ts
index 5e90f440f07..fe461fe6b7b 100644
--- a/src/vs/base/common/objects.ts
+++ b/src/vs/base/common/objects.ts
@@ -223,6 +223,11 @@ export function getOrDefault<T, R>(obj: T, fn: (obj: T) => R, defaultValue: R |
 	return typeof result === 'undefined' ? defaultValue : result;
 }
 
+export function getOrDefault2<T, R>(obj: T, fn: (obj: T) => R | undefined, defaultValue: R): R {
+	const result = fn(obj);
+	return typeof result === 'undefined' ? defaultValue : result;
+}
+
 type obj = { [key: string]: any; };
 /**
  * Returns an object that has keys for each value that is different in the base object. Keys