after upgrading from 5.4.2 to 6.0.6 in the CP I get the error
Warning Attempt to read property "group_name" on null ee/ExpressionEngine/Model/Template/Template.php, line 118
What does that mean and how could it be fixed? Main parts of the website (frontend) seem to run ok.
Thanks in advance for any advice!!!
EDIT: I found that the error appears when I edit a template. Editing the template via CP or via FTP-file works, also creating and deleting template groups. What could this be?