Нельзя задать Visibility или вызвать Show или ShowDialog после закрытия окна.

в System.Windows.Window.VerifyCanShow()
в System.Windows.Window.Show()
в AvalonDock.DockingManager.OnDragEnter(Point point)
в AvalonDock.DragPaneServices.StartDrag(FloatingWindow wnd, Point point, Point offset)
в AvalonDock.DockingManager.Drag(FloatingWindow floatingWindow, Point point, Point offset)
в AvalonDock.DockingManager.Drag(DocumentContent documentContent, Point point, Point offset)
в AvalonDock.DocumentContent.OnDragStart(Point ptMouse, Point ptRelativeMouse)
в AvalonDock.ManagedContent.OnDragMouseLeave(Object sender, MouseEventArgs e)
в System.Windows.Input.MouseEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
в System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
в System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
в System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
в System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
в System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
в System.Windows.MouseOverProperty.FireNotifications(UIElement uie, ContentElement ce, UIElement3D uie3D, Boolean oldValue)
в System.Windows.ReverseInheritProperty.FirePropertyChangeInAncestry(DependencyObject element, Boolean oldValue, DeferredElementTreeState treeState)
в System.Windows.ReverseInheritProperty.FirePropertyChangeInAncestry(DependencyObject element, Boolean oldValue, DeferredElementTreeState treeState)
в System.Windows.ReverseInheritProperty.FirePropertyChangeInAncestry(DependencyObject element, Boolean oldValue, DeferredElementTreeState treeState)
в System.Windows.ReverseInheritProperty.FirePropertyChangeInAncestry(DependencyObject element, Boolean oldValue, DeferredElementTreeState treeState)
в System.Windows.ReverseInheritProperty.FirePropertyChangeInAncestry(DependencyObject element, Boolean oldValue, DeferredElementTreeState treeState)
в System.Windows.ReverseInheritProperty.OnOriginValueChanged(DependencyObject oldOrigin, DependencyObject newOrigin, DeferredElementTreeState& oldTreeState)
в System.Windows.Input.MouseDevice.ChangeMouseOver(IInputElement mouseOver, Int32 timestamp)
в System.Windows.Input.MouseDevice.PreNotifyInput(Object sender, NotifyInputEventArgs e)
в System.Windows.Input.InputManager.ProcessStagingArea()
в System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
в System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
в System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
в System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
в System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
в MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
в MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
в System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
в System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)


Возникает на ровном месте :-/