Solve it with this post: https://forums.modx.com/thread/92644/migxdb---cmp-undefined
J had the same "undefined" problem...
My solution was to use the **exactly** same name for
* MIGX -> Settings -> Name
* MIGX -> Settings -> unique MIGX ID
* MIGX -> MIGXdb -> Classname
when the "MIGX -> Settings -> Name" and the "MIGX -> Settings -> unique MIGX ID" is different, the undefined error appears for me ...
Weirs but works, GOOD!