Make sure mParent is not null before using it

Otherwise, there will be NullPointerException.

Fix: 204883084
Test: Run the following code and see if it crashes.
  ViewGroup v = new FrameLayout(context);
  v.requestTransparentRegion(v);
  getWindowManager().addView(v, attrs);
  getWindowManager().removeView(v);
Change-Id: I059e13d2a258cf85dbc1694fb79b6088199de3fb