[Bug] FloatingWindow is created twice

[Khaos66]

I have a strange issue with a restored layout that contains a floating window.
It looks like DockingManager.DockingManager_Loaded is called twice and calls the CreateUIElementForModel method twice for the same FloatingWindows.
The callstack of the second time is long, but includes the first call of CreateUIElementForModel (see below).

There is a comment above this code that suggests, that this was an issue before. But I don't think that it fixes the issue.

// load floating windows not already loaded! (issue #59)
var items = new List<LayoutFloatingWindow>(Layout.FloatingWindows.Where(fw => !_fwList.Any(fwc => fwc.Model == fw)));
foreach (var fw in items)
	_fwList.Add(CreateUIElementForModel(fw) as LayoutFloatingWindowControl);

The Add is not executed before the second call to DockingManager_Loaded occurs. So the list is still empty and a second UIElement will be created for the same model. A better way would be to fill the List inside CreateUIElementForModel right after the element was created. That way the list will include the element when DockingManager_Loaded occurs again.

I'll create a PR for this ;)

Here is the full callstack by the way. I marked the important lines with '> '

> AvalonDock.dll!AvalonDock.DockingManager.DockingManager_Loaded(object sender, System.Windows.RoutedEventArgs e) Line 1962
	at E:\GitHub\AvalonDock\source\Components\AvalonDock\DockingManager.cs(1962)
PresentationCore.dll!System.Windows.RoutedEventHandlerInfo.InvokeHandler(object target, System.Windows.RoutedEventArgs routedEventArgs)
PresentationCore.dll!System.Windows.EventRoute.InvokeHandlersImpl(object source, System.Windows.RoutedEventArgs args, bool reRaised)
PresentationCore.dll!System.Windows.UIElement.RaiseEventImpl(System.Windows.DependencyObject sender, System.Windows.RoutedEventArgs args)
PresentationCore.dll!System.Windows.UIElement.RaiseEvent(System.Windows.RoutedEventArgs e)
PresentationFramework.dll!System.Windows.BroadcastEventHelper.BroadcastEvent(System.Windows.DependencyObject root, System.Windows.RoutedEvent routedEvent)
PresentationFramework.dll!System.Windows.BroadcastEventHelper.BroadcastLoadedEvent(object root)
PresentationCore.dll!MS.Internal.LoadedOrUnloadedOperation.DoWork()
PresentationCore.dll!System.Windows.Media.MediaContext.FireLoadedPendingCallbacks()
PresentationCore.dll!System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
PresentationCore.dll!System.Windows.Media.MediaContext.RenderMessageHandlerCore(object resizedCompositionTarget)
PresentationCore.dll!System.Windows.Media.MediaContext.RenderMessageHandler(object resizedCompositionTarget)
PresentationCore.dll!System.Windows.Media.MediaContext.Resize(System.Windows.Media.ICompositionTarget resizedCompositionTarget)
PresentationCore.dll!System.Windows.Interop.HwndTarget.OnResize()
PresentationCore.dll!System.Windows.Interop.HwndTarget.HandleMessage(MS.Internal.Interop.WindowMessage msg, System.IntPtr wparam, System.IntPtr lparam)
PresentationCore.dll!System.Windows.Interop.HwndSource.HwndTargetFilterMessage(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam, ref bool handled)
WindowsBase.dll!MS.Win32.HwndWrapper.WndProc(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam, ref bool handled)
WindowsBase.dll!MS.Win32.HwndSubclass.DispatcherCallbackOperation(object o)
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate callback, object args, int numArgs)
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.TryCatchWhen(object source, System.Delegate callback, object args, int numArgs, System.Delegate catchHandler)
WindowsBase.dll!System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority priority, System.TimeSpan timeout, System.Delegate method, object args, int numArgs)
WindowsBase.dll!MS.Win32.HwndSubclass.SubclassWndProc(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam)
[Native to Managed Transition]
[Managed to Native Transition]
WindowsBase.dll!MS.Win32.HwndSubclass.DefWndProcWrapper(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam)
[Native to Managed Transition]
[Managed to Native Transition]
WindowsBase.dll!MS.Win32.HwndSubclass.SubclassWndProc(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam)
[Native to Managed Transition]
[Managed to Native Transition]
PresentationFramework.dll!System.Windows.Window.Flush()
PresentationFramework.dll!System.Windows.Window.HwndStyleManager.System.IDisposable.Dispose()
PresentationFramework.dll!System.Windows.Window.CreateSourceWindow(bool duringShow)
PresentationFramework.dll!System.Windows.Window.CreateSourceWindowDuringShow()
PresentationFramework.dll!System.Windows.Window.SafeCreateWindowDuringShow()
PresentationFramework.dll!System.Windows.Window.ShowHelper(object booleanBox)
> PresentationFramework.dll!System.Windows.Window.Show()
> AvalonDock.dll!AvalonDock.DockingManager.CreateUIElementForModel(AvalonDock.Layout.ILayoutElement model) Line 1617
	at E:\GitHub\AvalonDock\source\Components\AvalonDock\DockingManager.cs(1617)
> AvalonDock.dll!AvalonDock.DockingManager.DockingManager_Loaded(object sender, System.Windows.RoutedEventArgs e) Line 1963
	at E:\GitHub\AvalonDock\source\Components\AvalonDock\DockingManager.cs(1963)
PresentationCore.dll!System.Windows.RoutedEventHandlerInfo.InvokeHandler(object target, System.Windows.RoutedEventArgs routedEventArgs)
PresentationCore.dll!System.Windows.EventRoute.InvokeHandlersImpl(object source, System.Windows.RoutedEventArgs args, bool reRaised)
PresentationCore.dll!System.Windows.UIElement.RaiseEventImpl(System.Windows.DependencyObject sender, System.Windows.RoutedEventArgs args)
PresentationCore.dll!System.Windows.UIElement.RaiseEvent(System.Windows.RoutedEventArgs e)
PresentationFramework.dll!System.Windows.BroadcastEventHelper.BroadcastEvent(System.Windows.DependencyObject root, System.Windows.RoutedEvent routedEvent)
PresentationFramework.dll!System.Windows.BroadcastEventHelper.BroadcastLoadedEvent(object root)
PresentationCore.dll!MS.Internal.LoadedOrUnloadedOperation.DoWork()
PresentationCore.dll!System.Windows.Media.MediaContext.FireLoadedPendingCallbacks()
PresentationCore.dll!System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
PresentationCore.dll!System.Windows.Media.MediaContext.RenderMessageHandlerCore(object resizedCompositionTarget)
PresentationCore.dll!System.Windows.Media.MediaContext.RenderMessageHandler(object resizedCompositionTarget)
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate callback, object args, int numArgs)
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.TryCatchWhen(object source, System.Delegate callback, object args, int numArgs, System.Delegate catchHandler)
WindowsBase.dll!System.Windows.Threading.DispatcherOperation.InvokeImpl()
WindowsBase.dll!System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(object state)
mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state)
WindowsBase.dll!MS.Internal.CulturePreservingExecutionContext.Run(MS.Internal.CulturePreservingExecutionContext executionContext, System.Threading.ContextCallback callback, object state)
WindowsBase.dll!System.Windows.Threading.DispatcherOperation.Invoke()
WindowsBase.dll!System.Windows.Threading.Dispatcher.ProcessQueue()
WindowsBase.dll!System.Windows.Threading.Dispatcher.WndProcHook(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam, ref bool handled)
WindowsBase.dll!MS.Win32.HwndWrapper.WndProc(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam, ref bool handled)
WindowsBase.dll!MS.Win32.HwndSubclass.DispatcherCallbackOperation(object o)
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate callback, object args, int numArgs)
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.TryCatchWhen(object source, System.Delegate callback, object args, int numArgs, System.Delegate catchHandler)
WindowsBase.dll!System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority priority, System.TimeSpan timeout, System.Delegate method, object args, int numArgs)
WindowsBase.dll!MS.Win32.HwndSubclass.SubclassWndProc(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam)
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