title: findFirstVisibleItemPositions doesn't work for recycleview android
body: <p>I have this code:</p>
  
  <pre><code>mRecycleView.setOnScrollListener(new RecyclerView.OnScrollListener() {
      @Override
      public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
          super.onScrolled(recyclerView, dx, dy);
          int[] firstVisibleItemPositions = new int[getResources().getInteger(R.integer.news_numbers_column)]; 
          int pastVisiblesItems = ((StaggeredGridLayoutManager)getLayoutManager()).findFirstVisibleItemPositions(firstVisibleItemPositions)[0];
      }
  });
  </code></pre>
  
  <p>When I scroll my recycleview I get this error:</p>
  
  <pre><code>java.lang.NullPointerException: Attempt to invoke virtual method 'int android.support.v7.widget.OrientationHelper.getStartAfterPadding()' on a null object reference
  at android.support.v7.widget.StaggeredGridLayoutManager$Span.findOneVisibleChild(StaggeredGridLayoutManager.java:2177)
  at android.support.v7.widget.StaggeredGridLayoutManager$Span.findFirstVisibleItemPosition(StaggeredGridLayoutManager.java:2153)
  at android.support.v7.widget.StaggeredGridLayoutManager.findFirstVisibleItemPositions(StaggeredGridLayoutManager.java:806)
  at info.lanouvelletribune.apps.android.ui.fragments.NewsFragment$1.onScrollStateChanged(NewsFragment.java:90)
  at android.support.v7.widget.RecyclerView.setScrollState(RecyclerView.java:751)
  at android.support.v7.widget.RecyclerView.onInterceptTouchEvent(RecyclerView.java:1535)
  at com.github.ksoichiro.android.observablescrollview.ObservableRecyclerView.onInterceptTouchEvent(ObservableRecyclerView.java:188)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2060)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2431)
  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2173)
  at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2390)
  at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1741)
  at android.app.Activity.dispatchTouchEvent(Activity.java:2826)
  at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2351)
  at android.view.View.dispatchPointerEvent(View.java:8590)
  at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4098)
  at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3964)
  at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3488)
  at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3541)
  at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3507)
  at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3515)
  at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3488)
  at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3541)
  at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3507)
  at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3617)
  at android
  </code></pre>
  
  <p>How can I do to handle this error and get the first visible item position?</p>