Steps to reproduce:

1. Add a action which contains (at least) the condition hostgroup=="hostgroup" or hostgroup<>"hostgroup"
2. make sure the action is enabled
3. delete the used hostgroup from configuration --> hostgroups

Result:
The action is now disabled and the hostgroup is gone

Expected:
The hostgroup should be removed from the condition-list, but the actions should still be active

Note:

The reason we delete hostgroups which are present in actions is the following: it's easier to remove a hostgroup from many (all) hosts by removing the whole hostgroup and re-creating it empty.