title: Why can't ButterKnife bind fields that are in private inner classes?
body: <p>In a fragment, I have a button that opens up a PopupWindow. </p>
  
  <pre><code>private class onMenuClickListener implements View.OnClickListener {
      @BindView(R.id.popup_radiogroup) RadioGroup popupRadioGroup;
      @BindView(R.id.popup_textview) TextView popupTextView;
  
      PopupWindow popupWindow = getPopupWindow(R.layout.popup_window);
  
      @Override
      public void onClick(View v) {
          ButterKnife.bind(this, popupWindow.getContentView());
          popupWindow.showAsDropDown(menuButton);
      }
  }
  private PopupWindow getPopupWindow(int layout_resource_id)  {
      LayoutInflater inflater = (LayoutInflater) getContext()
              .getSystemService(LAYOUT_INFLATER_SERVICE);
      View popupView = inflater.inflate(layout_resource_id,(ViewGroup)getView());
  
      return new PopupWindow(popupView,
              LinearLayout.LayoutParams.WRAP_CONTENT,
              LinearLayout.LayoutParams.WRAP_CONTENT,true);
  }
  </code></pre>
  
  <p>When I try to run this code, I get this error: "@BindView fields may not be contained in private classes."
  How come ButterKnife can't access private inner classes, but it can freely access protected ones?</p>