使用Avalon控件出现指定的参数已超出有效值的范围异常

使用Avalon控件出现指定的参数已超出有效值的范围错误
刚刚接触WPF，做了一个主窗口包含4个悬浮窗口的窗体，子窗体可以Auto Hide，但是当从Auto Hide变为固定的时候就会出现指定的参数已超出有效值的范围错误，具体错误信息如下，求高手指教。。。
未处理 System.ArgumentOutOfRangeException
  Message=指定的参数已超出有效值的范围。
参数名: index
  Source=PresentationFramework
  ParamName=index
  StackTrace:
       在 System.Windows.Controls.ItemCollection.GetItemAt(Int32 index)
       在 AvalonDock.DockingManager.ToggleAutoHide(DockablePane pane)
       在 AvalonDock.FlyoutDockablePane.ToggleAutoHide()
       在 AvalonDock.DockablePane.OnExecuteCommand(Object sender, ExecutedRoutedEventArgs e)
       在 System.Windows.Input.CommandBinding.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
       在 System.Windows.Input.CommandManager.ExecuteCommandBinding(Object sender, ExecutedRoutedEventArgs e, CommandBinding commandBinding)
       在 System.Windows.Input.CommandManager.FindCommandBinding(CommandBindingCollection commandBindings, Object sender, RoutedEventArgs e, ICommand command, Boolean execute)
       在 System.Windows.Input.CommandManager.FindCommandBinding(Object sender, RoutedEventArgs e, ICommand command, Boolean execute)
       在 System.Windows.Input.CommandManager.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
       在 System.Windows.UIElement.OnExecutedThunk(Object sender, ExecutedRoutedEventArgs e)
       在 System.Windows.Input.ExecutedRoutedEventArgs.InvokeEventHandler(Delegate genericHandler, Object target)
       在 System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
       在 System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
       在 System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
       在 System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
       在 System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
       在 System.Windows.Input.RoutedCommand.ExecuteImpl(Object parameter, IInputElement target, Boolean userInitiated)
       在 System.Windows.Input.RoutedCommand.ExecuteCore(Object parameter, IInputElement target, Boolean userInitiated)
       在 MS.Internal.Commands.CommandHelpers.CriticalExecuteCommandSource(ICommandSource commandSource, Boolean userInitiated)
       在 System.Windows.Controls.Primitives.ButtonBase.OnClick()
       在 System.Windows.Controls.Button.OnClick()
       在 System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e)
       在 System.Windows.UIElement.OnMouseLeftButtonUpThunk(Object sender, MouseButtonEventArgs e)
       在 System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)