Add nullcheck for problem of testcase in auditlog.

Testcase.probid is nullable, so the problem doesn't have to exist.

(cherry picked from commit 990838b)

Author: nickygerritsen

webapp/src/Controller/Jury/AuditLogController.php

@@ -168,7 +168,7 @@ private function generateDatatypeUrl(string $type, $id): ?string
                 return $this->generateUrl('jury_user', ['userId' => $id]);
             case 'testcase':
                 $testcase = $this->em->getRepository(Testcase::class)->find($id);
-                if ($testcase) {
+                if ($testcase && $testcase->getProblem()) {
                     return $this->generateUrl('jury_problem_testcases', ['probId' => $testcase->getProblem()->getProbid()]);
                 }
                 break;