In JForms XML file, change the type of the field.
Accesslevel is a control on its own.
It means it autopopulate its own data which is easier.
But I do not like this way, we are not able to change the aspect of the control.
This is the meaning of JDom : Separating content from aspect.
For accesslevel I wanted to do it but for the moment the SQL query to get the datas is still called by JDom.
It is still handled this old way for the moment, and I think it is well enough. If you want to change the aspect, for the JDom class.
Your problem was because the builder is proposing another control, so when you save the properties : boom.
Must fix that.