fix notebook tests

src/vs/workbench/api/common/extHostNotebook.ts

@@ -816,9 +816,9 @@ export class ExtHostNotebookController implements ExtHostNotebookShape, ExtHostN
 
 		this._notebookContentProviders.set(viewType, { extension, provider });
 
-		const listener = provider.onDidChangeNotebook(e => {
-			this._proxy.$onNotebookChange(viewType, e.document.uri);
-		});
+		const listener = provider.onDidChangeNotebook
+			? provider.onDidChangeNotebook(e => this._proxy.$onNotebookChange(viewType, e.document.uri))
+			: Disposable.None;
 
 		this._proxy.$registerNotebookProvider({ id: extension.identifier, location: extension.extensionLocation }, viewType, provider.kernel ? { id: viewType, label: provider.kernel.label, extensionLocation: extension.extensionLocation, preloads: provider.kernel.preloads } : undefined);
 		return new extHostTypes.Disposable(() => {