Rename stray this.refGroup usage to this.group in MWReferenceModel

this.refGroup is only ever read and never written, so obviously it's
always undefined. And it turns out that creating references with
listGroup='mwReference/undefined' crashes when you try to remove
them again.

Bug: 63085
Change-Id: Id326208bd6b3fe3b602ed6e3b57b7ea06d8ceb2c
Merge "Rename stray this.refGroup usage to this.group in MWReferenceModel"