diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index fa5442f0e51..8ef7ef697ab 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -397,9 +397,30 @@
 		"./vs/vscode.proposed.d.ts",
 		"./vs/workbench/api/common/configurationExtensionPoint.ts",
 		"./vs/workbench/api/common/menusExtensionPoint.ts",
+		"./vs/workbench/api/electron-browser/extHostCustomers.ts",
+		"./vs/workbench/api/electron-browser/mainThreadClipboard.ts",
+		"./vs/workbench/api/electron-browser/mainThreadCommands.ts",
+		"./vs/workbench/api/electron-browser/mainThreadErrors.ts",
+		"./vs/workbench/api/electron-browser/mainThreadFileSystem.ts",
+		"./vs/workbench/api/electron-browser/mainThreadLanguages.ts",
+		"./vs/workbench/api/electron-browser/mainThreadLogService.ts",
+		"./vs/workbench/api/electron-browser/mainThreadStatusBar.ts",
+		"./vs/workbench/api/electron-browser/mainThreadTelemetry.ts",
+		"./vs/workbench/api/electron-browser/mainThreadUrls.ts",
+		"./vs/workbench/api/electron-browser/mainThreadWindow.ts",
+		"./vs/workbench/api/electron-browser/mainThreadWorkspace.ts",
+		"./vs/workbench/api/node/extHost.protocol.ts",
+		"./vs/workbench/api/node/extHostClipboard.ts",
+		"./vs/workbench/api/node/extHostDialogs.ts",
 		"./vs/workbench/api/node/extHostExtensionActivator.ts",
+		"./vs/workbench/api/node/extHostHeapService.ts",
+		"./vs/workbench/api/node/extHostLogService.ts",
+		"./vs/workbench/api/node/extHostMessageService.ts",
 		"./vs/workbench/api/node/extHostSearch.fileIndex.ts",
+		"./vs/workbench/api/node/extHostStorage.ts",
 		"./vs/workbench/api/node/extHostTypes.ts",
+		"./vs/workbench/api/node/extHostUrls.ts",
+		"./vs/workbench/api/node/extHostWindow.ts",
 		"./vs/workbench/api/shared/editor.ts",
 		"./vs/workbench/api/shared/tasks.ts",
 		"./vs/workbench/browser/actions.ts",
diff --git a/src/vs/workbench/api/electron-browser/mainThreadWorkspace.ts b/src/vs/workbench/api/electron-browser/mainThreadWorkspace.ts
index 2b8311a2e3e..d99aab31ad5 100644
--- a/src/vs/workbench/api/electron-browser/mainThreadWorkspace.ts
+++ b/src/vs/workbench/api/electron-browser/mainThreadWorkspace.ts
@@ -103,7 +103,7 @@ export class MainThreadWorkspace implements MainThreadWorkspaceShape {
 	private _onDidChangeWorkspace(): void {
 		const workspace = this._contextService.getWorkbenchState() === WorkbenchState.EMPTY ? null : this._contextService.getWorkspace();
 		this._proxy.$acceptWorkspaceData(workspace ? {
-			configuration: workspace.configuration,
+			configuration: workspace.configuration || undefined,
 			folders: workspace.folders,
 			id: workspace.id,
 			name: this._labelService.getWorkspaceLabel(workspace)
@@ -112,7 +112,7 @@ export class MainThreadWorkspace implements MainThreadWorkspaceShape {
 
 	// --- search ---
 
-	$startFileSearch(includePattern: string, _includeFolder: UriComponents, excludePatternOrDisregardExcludes: string | false, maxResults: number, token: CancellationToken): Promise<URI[]> {
+	$startFileSearch(includePattern: string, _includeFolder: UriComponents, excludePatternOrDisregardExcludes: string | false, maxResults: number, token: CancellationToken): Promise<URI[]> | undefined {
 		const includeFolder = URI.revive(_includeFolder);
 		const workspace = this._contextService.getWorkspace();
 		if (!workspace.folders.length) {
@@ -196,7 +196,7 @@ export class MainThreadWorkspace implements MainThreadWorkspaceShape {
 		});
 	}
 
-	$resolveProxy(url: string): Promise<string> {
+	$resolveProxy(url: string): Promise<string | undefined> {
 		return this._windowService.resolveProxy(url);
 	}
 }
diff --git a/src/vs/workbench/api/node/extHost.protocol.ts b/src/vs/workbench/api/node/extHost.protocol.ts
index a66f695c65c..944adfb24f7 100644
--- a/src/vs/workbench/api/node/extHost.protocol.ts
+++ b/src/vs/workbench/api/node/extHost.protocol.ts
@@ -505,12 +505,12 @@ export interface ExtHostUrlsShape {
 }
 
 export interface MainThreadWorkspaceShape extends IDisposable {
-	$startFileSearch(includePattern: string, includeFolder: URI, excludePatternOrDisregardExcludes: string | false, maxResults: number, token: CancellationToken): Promise<UriComponents[]>;
+	$startFileSearch(includePattern: string, includeFolder: URI, excludePatternOrDisregardExcludes: string | false, maxResults: number, token: CancellationToken): Promise<UriComponents[]> | undefined;
 	$startTextSearch(query: IPatternInfo, options: ITextQueryBuilderOptions, requestId: number, token: CancellationToken): Promise<vscode.TextSearchComplete>;
 	$checkExists(includes: string[], token: CancellationToken): Promise<boolean>;
 	$saveAll(includeUntitled?: boolean): Promise<boolean>;
 	$updateWorkspaceFolders(extensionName: string, index: number, deleteCount: number, workspaceFoldersToAdd: { uri: UriComponents, name?: string }[]): Promise<void>;
-	$resolveProxy(url: string): Promise<string>;
+	$resolveProxy(url: string): Promise<string | undefined>;
 }
 
 export interface IFileChangeDto {
@@ -718,7 +718,7 @@ export interface ExtHostTreeViewsShape {
 }
 
 export interface ExtHostWorkspaceShape {
-	$acceptWorkspaceData(workspace: IWorkspaceData): void;
+	$acceptWorkspaceData(workspace: IWorkspaceData | null): void;
 	$handleTextSearchResult(result: IRawFileMatch2, requestId: number): void;
 }