code: @UiChild(limit = 1, tagname = "widget")
    public void setChildWidget(final TakesValue<T> pwidget) {
      widget = (Widget) pwidget;
      contents.add(widget);
      setEditor(new ExtendedValueBoxEditor<>(pwidget, this));
    }
docs: Set the widget that the EditorPanel will display. This method will automatically call
  {@link #setEditor}.
  
  @param pwidget a {@link IsEditor} widget