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src/vs/workbench/contrib/notebook/browser/notebookEditorWidget.ts

@@ -84,7 +84,7 @@ export class NotebookEditorWidget extends Disposable implements INotebookEditor
 	private _dimension: DOM.Dimension | null = null;
 	private _shadowElementViewInfo: { height: number, width: number, top: number; left: number; } | null = null;
 
-	private _cellListFocusTracker: DOM.IFocusTracker;
+	private _cellListFocusTracker: DOM.IFocusTracker | null = null;
 	private _editorFocus: IContextKey<boolean> | null = null;
 	private _cellListFocus: IContextKey<boolean> | null = null;
 	private _outputFocus: IContextKey<boolean> | null = null;