Get error message after manually updating a grader report

I installed Dataform in Moodle 2.7+.
In a Test course I have 3 Dataform activities, but not for grading.
When I open the grader report in another course without any Dataform activities, turn editing on, change a grade and save the grader report, I get the error message:
Fatal error: Class 'mod_dataform\observer\grade_item' not found in /home/pibceduc/public_html/mod/dataform/classes/observer/completion.php on line 94
Of course this error message only appears when Debugging is on. Without Debugging turned on, the page of the saved Grader report is short and looks like this:
https://www.dropbox.com/s/z2ijypk84z99yut/Grader%20report%20without%20error%20message.jpg?dl=0