diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index abecf8c05d1..f2f69af3270 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -741,6 +741,7 @@
 		"./vs/workbench/services/themes/common/colorThemeSchema.ts",
 		"./vs/workbench/services/themes/common/fileIconThemeSchema.ts",
 		"./vs/workbench/services/themes/common/workbenchThemeService.ts",
+		"./vs/workbench/services/themes/electron-browser/fileIconThemeData.ts",
 		"./vs/workbench/services/title/common/titleService.ts",
 		"./vs/workbench/services/workspace/common/workspaceEditing.ts",
 		"./vs/workbench/test/electron-browser/api/mock.ts"
diff --git a/src/vs/workbench/services/themes/common/workbenchThemeService.ts b/src/vs/workbench/services/themes/common/workbenchThemeService.ts
index 71d5dd447e1..5fa7344a2d4 100644
--- a/src/vs/workbench/services/themes/common/workbenchThemeService.ts
+++ b/src/vs/workbench/services/themes/common/workbenchThemeService.ts
@@ -41,9 +41,9 @@ export interface IColorMap {
 export interface IFileIconTheme extends IIconTheme {
 	readonly id: string;
 	readonly label: string;
-	readonly settingsId: string;
+	readonly settingsId?: string;
 	readonly description?: string;
-	readonly extensionData: ExtensionData;
+	readonly extensionData?: ExtensionData;
 
 	readonly isLoaded: boolean;
 	readonly hasFileIcons: boolean;
diff --git a/src/vs/workbench/services/themes/electron-browser/fileIconThemeData.ts b/src/vs/workbench/services/themes/electron-browser/fileIconThemeData.ts
index 5b4136989fb..bc1449601f0 100644
--- a/src/vs/workbench/services/themes/electron-browser/fileIconThemeData.ts
+++ b/src/vs/workbench/services/themes/electron-browser/fileIconThemeData.ts
@@ -15,25 +15,24 @@ import { getParseErrorMessage } from 'vs/base/common/jsonErrorMessages';
 export class FileIconThemeData implements IFileIconTheme {
 	id: string;
 	label: string;
-	settingsId: string;
+	settingsId?: string;
 	description?: string;
 	hasFileIcons: boolean;
 	hasFolderIcons: boolean;
 	hidesExplorerArrows: boolean;
 	isLoaded: boolean;
 	location?: URI;
-	extensionData: ExtensionData;
+	extensionData?: ExtensionData;
 
 	styleSheetContent?: string;
 
-	private constructor() {
-	}
+	private constructor() { }
 
 	public ensureLoaded(fileService: IFileService): Thenable<string> {
 		if (!this.isLoaded) {
 			if (this.location) {
 				return _loadIconThemeDocument(fileService, this.location).then(iconThemeDocument => {
-					let result = _processIconThemeDocument(this.id, this.location, iconThemeDocument);
+					let result = _processIconThemeDocument(this.id, this.location!, iconThemeDocument);
 					this.styleSheetContent = result.content;
 					this.hasFileIcons = result.hasFileIcons;
 					this.hasFolderIcons = result.hasFolderIcons;
@@ -66,17 +65,17 @@ export class FileIconThemeData implements IFileIconTheme {
 			themeData = FileIconThemeData._noIconTheme = new FileIconThemeData();
 			themeData.id = '';
 			themeData.label = '';
-			themeData.settingsId = null;
+			themeData.settingsId = undefined;
 			themeData.hasFileIcons = false;
 			themeData.hasFolderIcons = false;
 			themeData.hidesExplorerArrows = false;
 			themeData.isLoaded = true;
-			themeData.extensionData = null;
+			themeData.extensionData = undefined;
 		}
 		return themeData;
 	}
 
-	static fromStorageData(input: string): FileIconThemeData {
+	static fromStorageData(input: string): FileIconThemeData | null {
 		try {
 			let data = JSON.parse(input);
 			let theme = new FileIconThemeData();
@@ -169,7 +168,7 @@ function _loadIconThemeDocument(fileService: IFileService, location: URI): Thena
 
 function _processIconThemeDocument(id: string, iconThemeDocumentLocation: URI, iconThemeDocument: IconThemeDocument): { content: string; hasFileIcons: boolean; hasFolderIcons: boolean; hidesExplorerArrows: boolean; } {
 
-	let result = { content: '', hasFileIcons: false, hasFolderIcons: false, hidesExplorerArrows: iconThemeDocument.hidesExplorerArrows };
+	const result = { content: '', hasFileIcons: false, hasFolderIcons: false, hidesExplorerArrows: !!iconThemeDocument.hidesExplorerArrows };
 
 	if (!iconThemeDocument.iconDefinitions) {
 		return result;
@@ -178,10 +177,10 @@ function _processIconThemeDocument(id: string, iconThemeDocumentLocation: URI, i
 
 	const iconThemeDocumentLocationDirname = resources.dirname(iconThemeDocumentLocation);
 	function resolvePath(path: string) {
-		return resources.joinPath(iconThemeDocumentLocationDirname, path);
+		return resources.joinPath(iconThemeDocumentLocationDirname!, path);
 	}
 
-	function collectSelectors(associations: IconsAssociation, baseThemeClassName?: string) {
+	function collectSelectors(associations: IconsAssociation | undefined, baseThemeClassName?: string) {
 		function addSelector(selector: string, defId: string) {
 			if (defId) {
 				let list = selectorByDefinitionId[defId];