title: FragmentPagerAdapter does not redraw its item Fragments and show blank layout
body: <p>My <code>fragment</code> (one of the parent fragments) has a <code>ViewPager</code>, and that <code>ViewPager</code> will load a number of child fragments by using <code>FragmentPagerAdapter</code>. </p>
  
  <p>I observed the child fragments will show properly first time after application run, and after that the child fragments will show blank after I move to other parent fragment and come back to that parent <code>fragment</code>. One of the child <code>fragment</code> will redraw properly if I swipe to the last child <code>fragment</code>.</p>