Fix: views might be null during onLayout.

added flag set to true during onLayout. The flag is used so we don't try to calculate dimensions of a view that might become null during layout (say if we are removing views).

I was getting this error when I removed views from the ScrollView's child and the get*ForViewRelativeOnlyChild methods were being called. During onLayout, the views that get*ForViewRelativeOnlyChild is trying to calculate the dimensions of might become null. So we should just return zero from get*ForViewRelativeOnlyChild during this time, as the info is likely garbage anyway.