The widget is not picking up the value for the enumeration correctly, this comes back as undefined.
This value is then checked against the allowed values and as undefined is not a valid value the validation is shown.
To work around this comment the line 122 out that reads:
this.checkEnumValue(this._contextObj.get[this.name]);
This is however not the solution as it seems that the widget does not update the attribute after clicking on the image.
I suggest to get in touch with the developer via the appstore.