Why can't ButterKnife bind fields that are in private inner classes?
<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>