prevent warnings in lt exercise

exercises/lt_exercise.php

@@ -81,7 +81,8 @@ class lt_exercise extends Exercise
 
             if ($musterLoesung === $studentSolution) {
                 $similarity_temp = 1;
-            } else if ($this->task['compare'] === 'levenshtein') {  // Levenshtein-Distanz
+            } else if (isset($this->task['compare'])) {
+                if ($this->task['compare'] === 'levenshtein') {  // Levenshtein-Distanz
                     $string1 = mb_substr($studentSolution, 0, 255);
                     $string2 = mb_substr($musterLoesung, 0, 255);
                     $divisor = max(mb_strlen($string1), mb_strlen($string2));
@@ -114,6 +115,7 @@ class lt_exercise extends Exercise
                         }
                     }
                 }
+            }
 
             if ($answer['score'] == 1) {  // correct
                 if ($similarity_temp > $similarity) {