RecycleView crashes in Fragment because LayoutManager is NULL

Question

I have three tabs who all in a tablayout, in Tab1, there is a RecycleView.

Today I want to add SwipeRefreshLayout to it:

<android.support.v4.widget.SwipeRefreshLayout
    android:id="@+id/swiperefreshlayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    </android.support.v7.widget.RecyclerView>

</android.support.v4.widget.SwipeRefreshLayout>

In Tab1Fragment:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.activity_main_list, container, false);
    recyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
    staggeredGridLayoutManager = new StaggeredGridLayoutManager(3, StaggeredGridLayoutManager.VERTICAL);
    recyclerView.setLayoutManager(staggeredGridLayoutManager); 
    swipeRefreshLayout = (SwipeRefreshLayout) view.findViewById(R.id.swiperefreshlayout);
    swipeRefreshLayout.setColorSchemeResources(android.R.color.holo_blue_bright, android.R.color.holo_green_light,
            android.R.color.holo_orange_light, android.R.color.holo_red_light);
    return view;
}

Unfortunately, When I scroll from tab1 to tab2, and back to tab1, the activity crashes:

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean  
android.support.v7.widget.RecyclerView$LayoutManager.canScrollVertically()' on a null object reference
  at android.support.v7.widget.RecyclerView.computeVerticalScrollOffset(RecyclerView.java:1613)
  at android.view.View.canScrollVertically(View.java:12880)

But obviously I had set LayoutManger! It appeared that the staggeredgridlayout is gone...

What more Weired is that, when I just click tab1, tab2, Activity does not crash, But when I scroll from tab2 to tab1, it crashed

What happened? Thank you!

PLUS the WHOLE error log:

12-02 16:57:59.857 28189-28189/xyz.doutu.doutu E/AndroidRuntime: Process: xyz.doutu.doutu, PID: 28189
        ava.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.support.v7.widget.RecyclerView$LayoutManager.canScrollVertically()' on a null object reference
  at android.support.v7.widget.RecyclerView.computeVerticalScrollOffset(RecyclerView.java:1613)
  at android.view.View.canScrollVertically(View.java:12880)
  at android.support.v4.view.ViewCompatICS.canScrollVertically(ViewCompatICS.java:35)
  at android.support.v4.view.ViewCompat$ICSViewCompatImpl.canScrollVertically(ViewCompat.java:1253)
  at android.support.v4.view.ViewCompat.canScrollVertically(ViewCompat.java:1695)
  at android.support.v4.widget.SwipeRefreshLayout.canChildScrollUp(SwipeRefreshLayout.java:646)
  at android.support.v4.widget.SwipeRefreshLayout.onInterceptTouchEvent(SwipeRefreshLayout.java:660)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1961)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2408)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2050)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2408)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2050)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2408)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2050)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2408)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2050)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2408)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2050)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2408)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2050)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2408)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2050)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2408)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2050)
  at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2390)
  at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1734)
  at android.app.Activity.dispatchTouchEvent(Activity.java:2833)
  at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:60)
  at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:60)
  at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2351)
  at android.view.View.dispatchPointerEvent(View.java:8700)

Answer 1

Move your recyclerView assignment to onViewCreated() instead of onCreateView(). The problem is when your TabFragment gets detached and re-attached on viewPager scroll.