Refreshing Grades from Turnitin error on a multi-part Turnitin assignment

[kiratskitizing]

Clicking the "Refresh all Grades from Turnitin" button on the settings page of a multi-part Turnitin assignment with submissions triggers an HTTP 500 error.

Steps to Reproduce:

1. Create a multi-part Turnitin assignment. (For single part assignment it's good)
2. Submit from at least 1 student. (Otherwise the button won't appear). Grade it.
3. Navigate to the settings page of this multi-part assignment.
4. Click "Refresh all Grades from Turnitin."
5. Observe the dev tool console and check the error.
6. (optional) In comparison, try the same button for a single-part assignment, and observe the console. There is no error.
   Expected Result: Grades are refreshed without any errors.

See gathered error message
error "Exception - count(): Argument #1 ($value) must be of type Countable|array, null given"
errorcode "generalexceptionmessage"

It is pointing to this line:

moodle-mod_turnitintooltwo/ajax.php

Line 248 in 18fe96e

$total = count($_SESSION["TiiSubmissions"][$part->id]);

[wooyunlong]

I have observed the issue too. I found that the 500 error occurs only in multi-part assignments where any part Has No Submissions, whether graded or not.

This scenario is quite common. For example, consider a three-part assignment. After the first part has been submitted, the lecturer might change the maximum marks for the second part. To update the gradebook with this new grade weighting, they would click on the sync button, assuming the gradebook reflects these changes. (#762)