Fix console error when adding a widget to a side panel (#6694)

* Fix console error when adding a widget to a side panel

* Remove child-added logic

packages/application/src/panelhandler.ts

@@ -38,26 +38,13 @@ export class PanelHandler {
  }
 
  /**
- * A message hook for child add/remove messages on the main area dock panel.
+ * A message hook for child remove messages on the panel handler.
  */
  private _panelChildHook = (
  handler: IMessageHandler,
  msg: Message
  ): boolean => {
  switch (msg.type) {
- case 'child-added':
- {
- const widget = (msg as Widget.ChildMessage).child;
- // If we already know about this widget, we're done
- if (this._items.find(v => v.widget === widget)) {
- break;
- }
-
- // Otherwise, add to the end by default
- const rank = this._items[this._items.length - 1].rank;
- this._items.push({ widget, rank });
- }
- break;
  case 'child-removed':
  {
  const widget = (msg as Widget.ChildMessage).child;