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

Source/devtools/front_end/sources/WatchExpressionsSidebarPane.js

Index: Source/devtools/front_end/sources/WatchExpressionsSidebarPane.js
diff --git a/Source/devtools/front_end/sources/WatchExpressionsSidebarPane.js b/Source/devtools/front_end/sources/WatchExpressionsSidebarPane.js
index f3575be0855ae3538c4c60ab8acd2ec3a4ad9df9..581d01f5f2e924e51870f33b4977cdd79432582f 100644
--- a/Source/devtools/front_end/sources/WatchExpressionsSidebarPane.js
+++ b/Source/devtools/front_end/sources/WatchExpressionsSidebarPane.js
@@ -107,11 +107,12 @@ WebInspector.WatchExpressionsSidebarPane.prototype = {
     },
 
     /**
-     * @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();
     },