use weakmap

Co-authored-by: Don Jayamanne <don.jayamanne@outlook.com>

src/notebooks/controllers/vscodeNotebookController.ts

@@ -97,7 +97,7 @@ export class VSCodeNotebookController implements Disposable, IVSCodeNotebookCont
         selected: boolean;
         notebook: NotebookDocument;
     }>();
-    private pendingCellAdditions = new Map<NotebookDocument, Promise<void>>();
+    private pendingCellAdditions = new WeakMap<NotebookDocument, Promise<void>>();
     private readonly _onDidDispose = new EventEmitter<void>();
     private readonly disposables: IDisposable[] = [];
     private notebookKernels = new WeakMap<NotebookDocument, IKernel>();