Fatal error: Call to private method assign::get_renderer() when uploading grading worksheet
This error is thrown as the result of calling a private function in assign/offline/locallib.php when uploading grading worksheet
The steps to replicate:

1. Create a new course with some enrolled students in it
2. Create a new assignment with setting 'Feedback settings > Offline grading worksheet' to Yes
3. Save and display the assignment
4. Click on 'View/grade all submissions'
5. Select 'Upload grading worksheet' from the dropdown menu
6. It gives you the above error message