void invalidateScrollingChild() {
        final View scrollingChild = findScrollingChild(mViewRef.get());
        mNestedScrollingChildRef = new WeakReference<>(scrollingChild);
    }
Default constructor for inflating ViewPagerBottomSheetBehaviors from layout.

@param context The {@link Context}.
@param attrs   The {@link AttributeSet}.
vpbs/src/main/java/biz/laenger/android/vpbs/ViewPagerBottomSheetBehavior.java