fix: do not raise exception when clearing multianswer answers in mult…

…iple tries mode

Businessmap: #taskid 47453

question.php

@@ -175,6 +175,12 @@ public function get_matching_answer(array $response) {
             }
 
             if ($this->parent) {
+                // Sometimes clearing existing answers call this method with the empty string when working with
+                // multiple tries. Do not throw exception in that case.
+                if ($answer == '') {
+                    return null;
+                }
+
                 // Questions with parent should be graded together in multianswerwiris qtype!
                 throw new moodle_exception('Questions with parent should be graded together');
             }