NullpointerException when trying to use UserGroup Methods

Hi,

I have a Report in which I have an Array of UserGroups and I would like to get the Groups UserList, but I always get a NullPointerException when trying to do so.

Using Methods inherited from Item works, but all UserGroup methods end up in a NullpointerException...

Do you have any ideas what I might be doing wrong?

Kind regards, Bennet