BR: ResizableWindow can double delete it's content component

[RolandMR]

If the Content Component calls a listener or callback from it’s destructor that then causes the Content Component to get cleared again, the ResizableWindow will double delete the content component.

ResizableWindow::clearContentComponent should be implemented like this:

tracktion: fixed double delete in ResizableWindow

[kunitoki]

or in a oneliner

delete std::exchange(contentComponent, nullptr);