I managed to reproduce the error. It is caused by a bug in the control -- the code "assumes" the group panel is always present. We have reworked the drag/drop grouping code and the bug does not appear in our latest dev builds.
We are working on publishing a second beta build release at the end of next week that will have that and the rest of the issues you reported fixed. In addition we will have a public property that will allow you to hide the group panel, so that you won't have to define the control template yourself.