Ensures isMainRequest method is available before call is made to allo…

…w deprecated isMasterRequest method to be called for Symfony 4

src/Validator/DataCollectingValidator.php

@@ -88,7 +88,9 @@ public function inContext(ExecutionContextInterface $context): ContextualValidat
 
     public function onKernelRequest(GetResponseEvent $event): void
     {
-        if ($event->isMainRequest()) {
+        if (method_exists($event, 'isMainRequest') && $event->isMainRequest()) {
+            $this->clearLastErrors();
+        } elseif ($event->isMasterRequest()) {
             $this->clearLastErrors();
         }
     }