NullPointerException android.support.v4.app.FragmentManagerImpl.saveFragmentBasicState

fragment 加 viewpager做tab的时候容易出这个空指针异常，看源码：

if (f.mSavedViewState != null) {
    if (result == null) {
        result = new Bundle();
    }
    result.putSparseParcelableArray(
            FragmentManagerImpl.VIEW_STATE_TAG, f.mSavedViewState);
}
if (!f.mUserVisibleHint) {
    if (result == null) {
        result = new Bundle();
    }
    // Only add this if it's not the default value
    result.putBoolean(FragmentManagerImpl.USER_VISIBLE_HINT_TAG, f.mUserVisibleHint);
}

不保存状态会进入第二个if抛空指针异常。

在fragment中加入如下代码，bug可解：

@Override 
public void onSaveInstanceState(Bundle outState) 
{
//first saving my state, so the bundle wont be empty.
outState.putString("KEY",  "WBUG_VALUE");
super.onSaveInstanceState(outState);
}

或者：

class MyFragment extends Fragment {

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        setUserVisibleHint(true);
    }

或者使用FragmentStatePagerAdapter替代  FragmentPageAdapter。