code: public void setEmptyText(CharSequence text) {
          ensureContent();
          if (mEmptyView != null && mEmptyView instanceof TextView) {
              ((TextView) mEmptyView).setText(text);
          } else {
              throw new IllegalStateException("Can't be used with a custom content view");
          }
      }
docs: The default content for a ProgressFragment has a TextView that can be shown when
  the content is empty {@link #setContentEmpty(boolean)}.
  If you would like to have it shown, call this method to supply the text it should use.
  
  @param text Text for empty view
  @see #setEmptyText(int)