Created Unassigned: Got NullReferenceException trying to dock aside [21682]

System.NullReferenceException: Object reference not set to an instance of an object.
в Xceed.Wpf.AvalonDock.Controls.DropTarget`1.<>c__DisplayClass1.<Drop>b__0()
в System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
в MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)