diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index d31d9b6ecd2d1..51a1f4d7baa55 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -194,6 +194,7 @@
 		"./vs/code/node/wait.ts",
 		"./vs/code/node/windowsFinder.ts",
 		"./vs/code/test/node/argv.test.ts",
+		"./vs/code/test/node/windowsFinder.test.ts",
 		"./vs/editor/contrib/bracketMatching/bracketMatching.ts",
 		"./vs/editor/contrib/bracketMatching/test/bracketMatching.test.ts",
 		"./vs/editor/contrib/caretOperations/caretOperations.ts",
diff --git a/src/vs/code/node/windowsFinder.ts b/src/vs/code/node/windowsFinder.ts
index 8541b69dbe562..4c1765657dee0 100644
--- a/src/vs/code/node/windowsFinder.ts
+++ b/src/vs/code/node/windowsFinder.ts
@@ -26,7 +26,7 @@ export interface IBestWindowOrFolderOptions<W extends ISimpleWindow> {
 	fileUri?: URI;
 	userHome?: string;
 	codeSettingsFolder?: string;
-	workspaceResolver: (workspace: IWorkspaceIdentifier) => IResolvedWorkspace;
+	workspaceResolver: (workspace: IWorkspaceIdentifier) => IResolvedWorkspace | null;
 }
 
 export function findBestWindowOrFolderForFile<W extends ISimpleWindow>({ windows, newWindow, reuseWindow, context, fileUri, workspaceResolver }: IBestWindowOrFolderOptions<W>): W | null {
@@ -39,7 +39,7 @@ export function findBestWindowOrFolderForFile<W extends ISimpleWindow>({ windows
 	return !newWindow ? getLastActiveWindow(windows) : null;
 }
 
-function findWindowOnFilePath<W extends ISimpleWindow>(windows: W[], fileUri: URI, workspaceResolver: (workspace: IWorkspaceIdentifier) => IResolvedWorkspace): W | null {
+function findWindowOnFilePath<W extends ISimpleWindow>(windows: W[], fileUri: URI, workspaceResolver: (workspace: IWorkspaceIdentifier) => IResolvedWorkspace | null): W | null {
 
 	// First check for windows with workspaces that have a parent folder of the provided path opened
 	const workspaceWindows = windows.filter(window => !!window.openedWorkspace);
diff --git a/src/vs/code/test/node/windowsFinder.test.ts b/src/vs/code/test/node/windowsFinder.test.ts
index 0c04c5b2cf544..bd9e7ee187afb 100644
--- a/src/vs/code/test/node/windowsFinder.test.ts
+++ b/src/vs/code/test/node/windowsFinder.test.ts
@@ -31,7 +31,7 @@ function options(custom?: Partial<IBestWindowOrFolderOptions<ISimpleWindow>>): I
 }
 
 const vscodeFolderWindow: ISimpleWindow = { lastFocusTime: 1, openedFolderUri: URI.file(path.join(fixturesFolder, 'vscode_folder')) };
-const lastActiveWindow: ISimpleWindow = { lastFocusTime: 3, openedFolderUri: null };
+const lastActiveWindow: ISimpleWindow = { lastFocusTime: 3, openedFolderUri: undefined };
 const noVscodeFolderWindow: ISimpleWindow = { lastFocusTime: 2, openedFolderUri: URI.file(path.join(fixturesFolder, 'no_vscode_folder')) };
 const windows: ISimpleWindow[] = [
 	vscodeFolderWindow,