Deprecation fix for SF4.2
Deprecation fix for SF4.2

A tree builder without a root node is deprecated since Symfony 4.2 and will not be supported anymore in 5.0.

more details:
symfony.com/blog/new-in-symfony-4-2-important-deprecations#deprecated-tree-builders-without-root-nodes

alcaeus/DoctrineMongoDBBundle,doctrine/DoctrineMongoDBBundle,malarzm/DoctrineMongoDBBundle,malarzm/DoctrineMongoDBBundle,alcaeus/DoctrineMongoDBBundle