Admin UI for List View uses non-existant "default" columnGroup

[sleidig]

If creating a new entity via Admin UI, the list view automatically gets "columnGroups": { "default": "Overview", "mobile": "Overview",. Unless the user actually leaves that tab name unchanged, the config ends up in a state where the list remains empty because it tries to load a column group as default that does not exist.

• --> use first columnGroup.groups entry as default instead of configuring that explicitly
• if "mobile" (maybe better "mobileDefaultGroup"? is not found, use the first group as fallback?