DevTools: do no throw exception in "add watch" context menu item

Source/devtools/front_end/sources/WatchExpressionsSidebarPane.js

 /*
  * Copyright (C) IBM Corp. 2009  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 89 matching lines @@
     _refreshExpressionsIfNeeded: function()
     {
         if (this._requiresUpdate && this.isShowing()) {
             this._rebuildWatchExpressions();
             delete this._requiresUpdate;
         } else
             this._requiresUpdate = true;
     },
 
     /**
-     * @param {!WebInspector.Event} event
+     * @param {!WebInspector.Event=} event
      */
     _addButtonClicked: function(event)
     {
-        event.consume(true);
+        if (event)
+            event.consume(true);
         this.expand();
         this._createWatchExpression(null).startEditing();
     },
 
     /**
      * @param {!WebInspector.Event} event
      */
     _refreshButtonClicked: function(event)
     {
         event.consume();
@@ skipping 298 matching lines @@
             contextMenu.appendApplicableItems(this._result);
     },
 
     _copyValueButtonClicked: function()
     {
         InspectorFrontendHost.copyText(this._valueElement.textContent);
     },
 
     __proto__: WebInspector.Object.prototype
 }