(MonoDevelop:15250): Gtk-CRITICAL **: gtk_object_destroy: assertion `object != NULL' failed

(MonoDevelop:15250): Gtk-CRITICAL **: gtk_widget_get_parent: assertion `GTK_IS_WIDGET (widget)' failed
System.NullReferenceException: Object reference not set to an instance of an object
  at Stetic.Editor.ActionMenu.Refresh () [0x00000]
  at Stetic.Editor.ActionMenu.OnChildAdded (System.Object ob, Stetic.Wrapper.ActionTreeNodeArgs args) [0x00000]
  at (wrapper delegate-invoke) System.MulticastDelegate:invoke_void_object_ActionTreeNodeArgs (object,Stetic.Wrapper.ActionTreeNodeArgs)
  at Stetic.Wrapper.ActionTreeNode.NotifyChildAdded (Stetic.Wrapper.ActionTreeNode node) [0x00000]
  at Stetic.Wrapper.ActionTreeNodeCollection.OnInsertComplete (Int32 index, System.Object val) [0x00000]
  at System.Collections.CollectionBase.System.Collections.IList.Insert (Int32 index, System.Object value) [0x00000]
Segmentation fault