It was working earlier, but I have created and deleted a group a couple time. Now when logged in as Administrator, when I click on Site Access, I get this error:

Fatal error: Call to a member function getGroupID() on a non-object in /home/content/U/S/S/USSDSA/html/updates/concrete5.6.0.2/concrete/core/controllers/single_pages/dashboard/system/permissions/site.php on line 38

Any ideas how to fix this?

Thanks!
Michael