[Fix] TransitioningContentControl crushes when a view attaches visual tree many times

[AuroraZiling]

Related Issue: #258

What Happened:

When adding some items after emptying the SukiSideMenu, it can cause TransitioningContentControl to crash if those added items ever appear.

Possible Causes:

The Views that were found to be causing the problem had their VisualParent in the TransitioningContentControl's PushContent() function not empty, but either FirstBuffer or SecondBuffer. because the VisualParent was reassigned to another Buffer when it was not empty, this would cause a crash.

Unhandled exception. System.InvalidOperationException: 
The control UotanToolbox.Features.Home.HomeView (Content = WrapPanel) 
already has a visual parent ContentPresenter (Name = PART_FirstBufferControl) 
while trying to add it as a child of ContentPresenter (Name = PART_SecondBufferControl).

I don't know if there will be any side effects from this pr, but this solves the problem