When holding down SHIFT and mouse left click dockItem header cause NullReferenceException.

[lhyzf]

在 DevZest.Windows.Docking.Primitives.DockManager.WindowHandler.get_NativeOverlayCanvas() 在 ...\WpfDocking\Src\Windows\Docking\Primitives\DockManager.WindowHandler.cs 中: 第 118 行

I comment out TestDrop method in IsShiftKeyDown property in file WpfDocking\Src\Windows\Docking\Primitives\DockManager.DragHandler.cs, it seems ok but I don't know if it has other impacts.

private bool IsShiftKeyDown
{
    set
    {
        bool oldValue = GetIsShiftKeyDown(DockControl);
        if (oldValue == value)
            return;

        SetIsShiftKeyDown(DockControl, value);
        //TestDrop();
    }
}