Fix failing test

src/vs/workbench/api/electron-browser/mainThreadDocumentsAndEditors.ts

@@ -230,7 +230,7 @@ class MainThreadDocumentAndEditorStateComputer {
 
 		// editor: only take those that have a not too large model
 		const editors = new Map<string, TextEditorSnapshot>();
-		let activeEditor: string | undefined = undefined;
+		let activeEditor: string | null = null; // Strict null work. This doesn't like being undefined!
 
 		for (const editor of this._codeEditorService.listCodeEditors()) {
 			if (editor.isSimpleWidget) {