Fix a compilation error.

packages/notebook/src/index.ts

@@ -4,10 +4,10 @@
 import '../style/index.css';
 
 export * from './actions';
-export * from './notebooktools';
 export * from './default-toolbar';
 export * from './model';
 export * from './modelfactory';
+export * from './notebooktools';
 export * from './panel';
 export * from './tracker';
 export * from './widget';

packages/notebook/src/notebooktools.ts

@@ -171,7 +171,6 @@ export class NotebookTools extends Widget implements INotebookTools {
    * Handle a change to the notebook panel.
    */
   private _onNotebookPanelChanged(): void {
-    const panel = this.notebookPanel;
     each(this._toolChildren(), widget => {
       MessageLoop.sendMessage(widget, NotebookTools.NotebookPanelMessage);
     });