Handle null uris in content changed events.

        @Override

        public void onChange(boolean selfChange, Uri uri) {

            if (uri == null) {

                Log.w(TAG, "Received onChange event for null uri. Skipping.");

                return;

            }

            if (DEBUG) Log.d(TAG, "Updating roots due to change at " + uri);

            updateAuthorityAsync(uri.getAuthority());

        }