code: @Nullable
      @SuppressWarnings("unchecked")
      public <T extends Fragment> T findFragmentByPane(@IntRange(from = 1, to = 2) int pane) {
          String tag = Elements.get(sPanes, pane - 1);
          return tag != null ? (T) getFragmentManager().findFragmentByTag(tag) : null;
      }
docs: Get the fragment that is displayed in the pane.
  
  @return null if a fragment hasn't been added to the pane yet