function callback(e) {
    var menuItem = e.target;
    if (menuItem) {
      var value = menuItem.getValue();
      if (thisField.sourceBlock_ && thisField.validator_) {
        // Call any validation function, and allow it to override.
        var override = thisField.validator_(value);
        if (override !== undefined) {
          value = override;
        }
      }
      if (value !== null) {
        thisField.setValue(value);
      }
    }
    Blockly.WidgetDiv.hideIfOwner(thisField);
  }
Create a dropdown menu under the text.
@private