Fix crash while animating removed stack

    public boolean setPinnedStackAlpha(float alpha) {

        // Hold the lock since this is called from the BoundsAnimator running on the UiThread

        synchronized (mWmService.mGlobalLock) {

            getPendingTransaction().setAlpha(getSurfaceControl(),

                    mCancelCurrentBoundsAnimation ? 1 : alpha);

            final SurfaceControl sc = getSurfaceControl();

            if (sc == null || !sc.isValid()) {

                // If the stack is already removed, don't bother updating any stack animation

                return false;

            }

            getPendingTransaction().setAlpha(sc, mCancelCurrentBoundsAnimation ? 1 : alpha);

            scheduleAnimation();

            return !mCancelCurrentBoundsAnimation;

        }