在fragment生命周期onActivityCreated中发生null pointer exception

@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
mSwipeToLoadLayout.post(new Runnable() {
@Override
public void run() {
// 上面调用mSwipeToLoadLayout正常，下面这行代码发生空指针异常
mSwipeToLoadLayout.setRefreshing(true);
}
});
}