Fixed accessing non-interface methods in StructWrapperValidator (#750)

For more details see #750

Key changes:

* Fixed accessing non-interface methods in StructWrapperValidator

Author: alongosz

src/contracts/Validation/StructWrapperValidator.php

@@ -62,8 +62,8 @@ public function validate($value, $constraints = null, $groups = null): Constrain
                 $error->getInvalidValue(),
                 $error->getPlural(),
                 $error->getCode(),
-                $error->getConstraint(),
-                $error->getCause()
+                $error instanceof ConstraintViolation ? $error->getConstraint() : null,
+                $error instanceof ConstraintViolation ? $error->getCause() : null
             );
 
             $unwrappedErrors->add($unwrappedError);