Resolve the 'Unknown property mappedBy' issue when the parent field i…

…s defined before the child field in a tree structure.

src/Tree/Hydrator/ORM/TreeObjectHydrator.php

@@ -249,7 +249,7 @@ protected function getChildrenField($entityClass)
  $associationMapping = $meta->getAssociationMapping($property->getName());
 
  // Make sure the association is mapped by the parent property
- if ($associationMapping['mappedBy'] !== $this->parentField) {
+ if (!isset($associationMapping['mappedBy']) || $associationMapping['mappedBy'] !== $this->parentField) {
  continue;
  }