When drag and drop a LayoutAnchorable from a side to the other side an object null exception had been threw.
Suggest to fix this issue is add 1 more null check to Xceed.Wpf.AvalonDock.Controls.DropTarget<T> method public void Drop(LayoutFloatingWindow floatingWindow) as below
Dispatcher.BeginInvoke(new Action(() =>
if (currentActiveContent == null) return;
currentActiveContent.IsSelected = false;
currentActiveContent.IsActive = false;
currentActiveContent.IsActive = true;