Commented Unassigned: [2.9.0] AvalonDock crash when undocking into AnchorableFloatingWindow [22450]

The situation I have is that at least two anchorables are stacked, and this combound is docked at one of the sides of the application, like:

In my actual application, I get a randomly appearing (but hardly reproducable) case where, when pulling this combound out of the Application (in an AnchorableFloatingWindow):
The window is correctly created, but directly looses its focus (so the dragging abruptly stops). When its header is then clicked (for example to drag the window), the application crashes at:

File: [LayoutAnchorableFloatingWindowControl.cs](https://wpftoolkit.codeplex.com/SourceControl/latest#Main/Source/ExtendedWPFToolkitSolution/Src/Xceed.Wpf.AvalonDock/Controls/LayoutAnchorableFloatingWindowControl.cs)
LineNr: 250
Line:
```
_model.Descendents().OfType<LayoutAnchorablePane>().First(p => p.ChildrenCount > 0 && p.SelectedContent != null).SelectedContent.IsActive = true;
```

with:
```
System.InvalidOperationException was unhandled
HResult=-2146233079
Message=Sequence contains no matching element
Source=System.Core
StackTrace:
at System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
at Xceed.Wpf.AvalonDock.Controls.LayoutAnchorableFloatingWindowControl.FilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) in LayoutAnchorableFloatingWindowControl.cs:line 250
at System.Windows.Interop.HwndSource.PublicHooksFilterMessage(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)
at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
at System.Windows.Application.RunDispatcher(Object ignore)
at System.Windows.Application.RunInternal(Window window)
at System.Windows.Application.Run(Window window)
at System.Windows.Application.Run()
##########################################################
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
InnerException:
```

The failing condition of the LINQ-Query is: `p.SelectedContent != null`

When the FloatingAnchorableWindow's tab is changed before clicking it's header (e.g. in the above screenshot from Panel2 to Panel3), the error does not appear.

Unfortunately, due to [3.0.0 AvalonDock MVVM binding defect](https://wpftoolkit.codeplex.com/workitem/22448), I can't test this with 3.0.0
The Test-Application used to generate the Screenshots does not produce the error at all.
Comments: ** Comment from web user: hiwatari_seiji **

I just found out what is causing this. I manually synchronized the Anchorable's `IsVisible`-Property to my ViewModel's `IsVisible` property. This binding ended up setting (not changing!) the property on the panel, while the combound was dragged out of the application.

I don't know if this can still be considered a bug or a usage error.