Fixed a bug in widget-stdmod

_fillHeight depends on this._currFillNode being set by _uiSetFillHeight
However, if the fillHeight node's content is set after render(),
_uiSetFillHeight doesn't set this._currFillNode, because the fill node doesn't exist (yet).

When the content is set, _fillHeight is called after('contentUpdate'), but since
this._currFillNode isn't set, it calls fillHeight with undefined.

The fix is to have _fillHeight getStdModNode() instead of relying on this._currFillNode