Permissions screen will not open when a domain refers to a non-existing group

When a hippo:domain has a reference to a group that no longer exists, the PermissionsPanel throws an overview when the CMS user wants to see the Permissions admin panel.
This is caused by the Group.forName() method.

Current reproduction path:

• Go into a hipposys:domain node
• Go into a hippo:authrole node below the hipposys:domain node
• Add a non-existing group name to the hipposys:groups property, e.g. "FRITS"

Current behavior: The permissions admin panel should no longer show and throw an exception in the system log

Expected behavior:

• The group permissions screen should show and the non-existing group should not be shown in the overview.