code: @Nullable
      public MentionSpan getMentionSpanAtOffset(int index) {
          MentionSpan[] spans = getSpans(index, index, MentionSpan.class);
          return (spans != null && spans.length > 0) ? spans[0] : null;
      }
docs: Given an integer offset, return the {@link MentionSpan} located at the offset in the text of the
  {@link android.widget.EditText}, if it exists. Otherwise, return null.
  
  @param index integer offset in text
  
  @return a {@link MentionSpan} located at index in text, or null