Fix: fixed exception on first comment added

Author: Marvin Kuhn

Classes/Service/CommentService.php

@@ -62,7 +62,10 @@ protected function createNode(NodeInterface $commentCollection, CommentDtoInterf
 
         if ($this->addCommentToTop) {
             $firstComment = $commentCollection->getChildNodes(null, 1);
-            $commentNode->moveBefore($firstComment[0]);
+            
+            if ($firstComment) {
+                $commentNode->moveBefore($firstComment[0]);
+            }
         }
 
         return $commentNode;