fix(Assignments): Minor fixes to get things to run

Signed-off-by: Marcel Klehr <mklehr@gmx.net>

Author: marcelklehr

lib/Service/AssignmentsService.php

@@ -78,6 +78,9 @@ public function createAssignment(?string $userId, string $prompt, int $startsAt,
 	public function runDueAssignmentsForUser(?string $userId) {
 		try {
 			foreach ($this->assignmentMapper->findDueAssignmentsForUser($userId) as $assignment) {
+				if ($assignment === null) {
+					continue;
+				}
 				$this->scheduleAssignmentRun($userId, $assignment->getId());
 			}
 		} catch (Exception $e) {