diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index b575f0a031a..6862624412d 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -97,8 +97,10 @@
 		"./vs/workbench/api/node/extHostDocuments.ts",
 		"./vs/workbench/api/node/extHostDocumentsAndEditors.ts",
 		"./vs/workbench/api/node/extHostExtensionActivator.ts",
+		"./vs/workbench/api/node/extHostFileSystem.ts",
 		"./vs/workbench/api/node/extHostFileSystemEventService.ts",
 		"./vs/workbench/api/node/extHostHeapService.ts",
+		"./vs/workbench/api/node/extHostLanguageFeatures.ts",
 		"./vs/workbench/api/node/extHostLanguages.ts",
 		"./vs/workbench/api/node/extHostLogService.ts",
 		"./vs/workbench/api/node/extHostMessageService.ts",
@@ -491,6 +493,7 @@
 		"./vs/workbench/services/untitled/common/untitledEditorService.ts",
 		"./vs/workbench/services/viewlet/browser/viewlet.ts",
 		"./vs/workbench/services/workspace/common/workspaceEditing.ts",
+		"./vs/workbench/services/workspace/node/workspaceEditingService.ts",
 		"./vs/workbench/test/browser/actionRegistry.test.ts",
 		"./vs/workbench/test/browser/viewlet.test.ts",
 		"./vs/workbench/test/common/editor/editorInput.test.ts",
@@ -504,6 +507,7 @@
 		"./vs/workbench/test/electron-browser/api/extHostDocumentSaveParticipant.test.ts",
 		"./vs/workbench/test/electron-browser/api/extHostDocumentsAndEditors.test.ts",
 		"./vs/workbench/test/electron-browser/api/extHostFileSystemEventService.test.ts",
+		"./vs/workbench/test/electron-browser/api/extHostLanguageFeatures.test.ts",
 		"./vs/workbench/test/electron-browser/api/extHostMessagerService.test.ts",
 		"./vs/workbench/test/electron-browser/api/extHostTextEditor.test.ts",
 		"./vs/workbench/test/electron-browser/api/extHostTextEditors.test.ts",
@@ -512,6 +516,7 @@
 		"./vs/workbench/test/electron-browser/api/extHostWebview.test.ts",
 		"./vs/workbench/test/electron-browser/api/extHostWorkspace.test.ts",
 		"./vs/workbench/test/electron-browser/api/mainThreadCommands.test.ts",
+		"./vs/workbench/test/electron-browser/api/mainThreadConfiguration.test.ts",
 		"./vs/workbench/test/electron-browser/api/mainThreadDiagnostics.test.ts",
 		"./vs/workbench/test/electron-browser/api/mainThreadDocumentContentProviders.test.ts",
 		"./vs/workbench/test/electron-browser/api/mock.ts",
diff --git a/src/vs/workbench/api/node/extHost.protocol.ts b/src/vs/workbench/api/node/extHost.protocol.ts
index 015c56d1e43..9b9f34f91ee 100644
--- a/src/vs/workbench/api/node/extHost.protocol.ts
+++ b/src/vs/workbench/api/node/extHost.protocol.ts
@@ -490,7 +490,7 @@ export interface WebviewPanelShowOptions {
 
 export interface MainThreadWebviewsShape extends IDisposable {
 	$createWebviewPanel(handle: WebviewPanelHandle, viewType: string, title: string, showOptions: WebviewPanelShowOptions, options: vscode.WebviewPanelOptions & vscode.WebviewOptions, extensionId: ExtensionIdentifier, extensionLocation: UriComponents): void;
-	$createWebviewCodeInset(handle: WebviewInsetHandle, symbolId: string, options: vscode.WebviewOptions, extensionLocation: UriComponents): void;
+	$createWebviewCodeInset(handle: WebviewInsetHandle, symbolId: string, options: vscode.WebviewOptions, extensionLocation: UriComponents | undefined): void;
 	$disposeWebview(handle: WebviewPanelHandle): void;
 	$reveal(handle: WebviewPanelHandle, showOptions: WebviewPanelShowOptions): void;
 	$setTitle(handle: WebviewPanelHandle, value: string): void;
diff --git a/src/vs/workbench/api/node/extHostLanguageFeatures.ts b/src/vs/workbench/api/node/extHostLanguageFeatures.ts
index cdc7a819b62..5d902dd3def 100644
--- a/src/vs/workbench/api/node/extHostLanguageFeatures.ts
+++ b/src/vs/workbench/api/node/extHostLanguageFeatures.ts
@@ -71,6 +71,7 @@ class DocumentSymbolAdapter {
 			const element = <modes.DocumentSymbol>{
 				name: info.name || '!!MISSING: name!!',
 				kind: typeConvert.SymbolKind.from(info.kind),
+				detail: undefined!, // Strict null override — avoid changing behavior
 				containerName: info.containerName,
 				range: typeConvert.Range.from(info.location.range),
 				selectionRange: typeConvert.Range.from(info.location.range),
@@ -85,7 +86,9 @@ class DocumentSymbolAdapter {
 				}
 				const parent = parentStack[parentStack.length - 1];
 				if (EditorRange.containsRange(parent.range, element.range) && !EditorRange.equalsRange(parent.range, element.range)) {
-					parent.children.push(element);
+					if (parent.children) {
+						parent.children.push(element);
+					}
 					parentStack.push(element);
 					break;
 				}
@@ -750,7 +753,7 @@ class SuggestAdapter {
 
 		} else if (item.insertText instanceof SnippetString) {
 			result.insertText = item.insertText.value;
-			result.insertTextRules |= modes.CompletionItemInsertTextRule.InsertAsSnippet;
+			result.insertTextRules! |= modes.CompletionItemInsertTextRule.InsertAsSnippet;
 
 		} else {
 			result.insertText = item.label;
@@ -1053,7 +1056,7 @@ export class ExtHostLanguageFeatures implements ExtHostLanguageFeaturesShape {
 		return ExtHostLanguageFeatures._handlePool++;
 	}
 
-	private _withAdapter<A, R>(handle: number, ctor: { new(...args: any[]): A }, callback: (adapter: A, extenson: IExtensionDescription) => Promise<R>): Promise<R> {
+	private _withAdapter<A, R>(handle: number, ctor: { new(...args: any[]): A }, callback: (adapter: A, extension: IExtensionDescription | undefined) => Promise<R>): Promise<R> {
 		const data = this._adapter.get(handle);
 		if (!data) {
 			return Promise.reject(new Error('no adapter found'));
@@ -1156,7 +1159,7 @@ export class ExtHostLanguageFeatures implements ExtHostLanguageFeaturesShape {
 		const webviewHandle = Math.random();
 		const webview = new ExtHostWebview(webviewHandle, this._webviewProxy, { enableScripts: true });
 		return this._withAdapter(handle, CodeInsetAdapter, async (adapter, extension) => {
-			await this._webviewProxy.$createWebviewCodeInset(webviewHandle, symbol.id, { enableCommandUris: true, enableScripts: true }, extension.extensionLocation);
+			await this._webviewProxy.$createWebviewCodeInset(webviewHandle, symbol.id, { enableCommandUris: true, enableScripts: true }, extension ? extension.extensionLocation : undefined);
 			return adapter.resolveCodeInset(symbol, webview, token);
 		});
 	}