Fix NPE in TaskSnapshotController

        if (top == null) {

            return null;

        }

        final WindowState mainWindow = top.findMainWindow();

        if (mainWindow == null) {

            return null;

        }

        final GraphicBuffer buffer = top.mDisplayContent.screenshotApplicationsToBuffer(top.token,

                -1, -1, false, 1.0f, false, true);

        if (buffer == null) {

            return null;

        }

        final WindowState mainWindow = top.findMainWindow();

        return new TaskSnapshot(buffer, top.getConfiguration().orientation,

                minRect(mainWindow.mContentInsets, mainWindow.mStableInsets), false /* reduced */,

                1f /* scale */);