Merge "viewcapture: fix NPE (WindowListener#mRoot)" into main

        @UiThread

        public void onDraw() {

            Trace.beginSection("vc#onDraw");

            captureViewTree(mRoot, mViewPropertyRef);

            try {

                View root = mRoot;

                if (root == null) {

                    // Handle the corner case where another (non-UI) thread

                    // concurrently stopped the capture and set mRoot = null

                    return;

                }

                captureViewTree(root, mViewPropertyRef);

                ViewPropertyRef captured = mViewPropertyRef.next;

                if (captured != null) {

                    captured.callback = mCaptureCallback;

                    mBgExecutor.execute(captured);

                }

                mIsFirstFrame = false;

            } finally {

                Trace.endSection();

            }

        }

        /**

         * Copy clean views from the last frame on the background thread. Clean views are