Fix Visibility issue on closing LayoutFloatingWindowControl

[scdmitryvodich]

The Issue is similar to one which we fixed with https://github.com/Dirkster99/AvalonDock/pull/36/files#diff-a23d38de9c85b74a8efe7ef17b582374L2403

The only difference is that at first time of Close of floating window called not from InternalClose but by handling WindowMessage.WM_CLOSE in System.Windows.Window.cs

It has the same call stack:
System.InvalidOperationException: Cannot set Visibility to Visible or call Show,
ShowDialog, Close, or WindowInteropHelper.EnsureHandle while a Window is closing.
at System.Windows.Window.VerifyNotClosing()
at System.Windows.Window.InternalClose(Boolean shutdown, Boolean ignoreCancel)
at Xceed.Wpf.AvalonDock.DockingManager.DockingManager_Unloaded(Object sender,
RoutedEventArgs e)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs
args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender,
RoutedEventArgs args)
at System.Windows.BroadcastEventHelper.BroadcastEvent(DependencyObject root,
RoutedEvent routedEvent)
at System.Windows.BroadcastEventHelper.BroadcastUnloadedEvent(Object root)
at MS.Internal.LoadedOrUnloadedOperation.DoWork()
at System.Windows.Media.MediaContext.FireLoadedPendingCallbacks()
at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object
resizedCompositionTarget)
at System.Windows.Media.MediaContext.RenderMessageHandler(Object
resizedCompositionTarget)
at System.Windows.Interop.HwndTarget.OnResize()
at System.Windows.Interop.HwndTarget.HandleMessage(WindowMessage msg, IntPtr
wparam, IntPtr lparam)
at System.Windows.Interop.HwndSource.HwndTargetFilterMessage(IntPtr hwnd, Int32
msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr
lParam, Boolean& handled)
at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback,
Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source,
Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

[Dirkster99]

Your change request makes sense to me. Thanks again.

There is a different issue/Pull Request where I am not sure wether thats a good fix (?) or not. But maybe I am too critical about it or get something wrong? Would you have a moment to review this and tell me your opinion?