NPE in Fragment.setUserVisibleHint() when using ViewPager

Question

I am at a loss for this. I am switching tabs manually in my ViewPager. I have this code inside my Activity: ``` @Override public void onBackPressed() { if (childFragmentStack.empty()) { // Go to the devices screen Intent intent = new Intent(this, SelectDeviceActivity.class); startActivity(intent); } else { Fragment fragment = childFragmentStack.pop(); if (fragment == null) { return; } processingBackStack = true; if (fragment instanceof ViewChildFragment) { viewFragment.activateFragment((ViewChildFragment) fragment); mViewPager.setCurrentItem(VIEW_FRAGMENT_INDEX, true); } else if (fragment instanceof SetupChildFragment) { setupFragment.activateFragment((SetupChildFragment) fragment); mViewPager.setCurrentItem(SETUP_FRAGMENT_INDEX, true); //** } else if (fragment == homeFragment) { mViewPager.setCurrentItem(HOME_FRAGMENT_INDEX, true); //** } processingBackStack = false; } } ``` If I scroll between tabs I add them to a Stack (the 'childFragmentStack'). I am using FragmentPagerAdapter to…

Accepted Answer

Finally! I'm now able to reliably recreate this error!

Another way that I found to recreate error is to close activity/app, and quickly reopen page with the ViewPager fragment. You may have to try a few times because in my tests I had to reopen the app within about 30ms. This time may be slower or faster for different speed devices.

The problem was that I only explicitly created the Fragment (using `new`) once, and kept a reference to that instance so that I could reuse it. One simple fix to this problem is to always return a `new` instance of the Fragment the `FragmentPagerAdapter.getItem(...)`, as shown below.

```
public class ViewPagerAdapter extends FragmentPagerAdapter {
    ...

@Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0: return mMyFragment; // Error. Has the edge-case crash.
            case 1: return new MyFragment(); // Works.
            default: return new MyDefaultFragment();
        }
    }
}
```

ps - The root problem likely has something to do with the Fragment lifecycle and trying to use it again while it's being destroyed.

pps - This solution fixes the NullPointerException for `android.app.Fragment.setUserVisibleHint(Fragment.java:997)` and should also work for `android.support.v4.app.Fragment.setUserVisibleHint`.

NPE in Fragment.setUserVisibleHint() when using ViewPager

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