Fix access to null $config variable (#2732)

Author: GromNaN

lib/Doctrine/ODM/MongoDB/DocumentManager.php

@@ -159,7 +159,7 @@ protected function __construct(?Client $client = null, ?Configuration $config =
             ],
         );
 
-        $this->classNameResolver = $config->isLazyGhostObjectEnabled()
+        $this->classNameResolver = $this->config->isLazyGhostObjectEnabled()
             ? new CachingClassNameResolver(new LazyGhostProxyClassNameResolver())
             : new CachingClassNameResolver(new ProxyManagerClassNameResolver($this->config));
 
@@ -186,8 +186,8 @@ protected function __construct(?Client $client = null, ?Configuration $config =
 
         $this->unitOfWork        = new UnitOfWork($this, $this->eventManager, $this->hydratorFactory);
         $this->schemaManager     = new SchemaManager($this, $this->metadataFactory);
-        $this->proxyFactory      = $config->isLazyGhostObjectEnabled()
-            ? new LazyGhostProxyFactory($this, $config->getProxyDir(), $config->getProxyNamespace(), $config->getAutoGenerateProxyClasses())
+        $this->proxyFactory      = $this->config->isLazyGhostObjectEnabled()
+            ? new LazyGhostProxyFactory($this, $this->config->getProxyDir(), $this->config->getProxyNamespace(), $this->config->getAutoGenerateProxyClasses())
             : new StaticProxyFactory($this);
         $this->repositoryFactory = $this->config->getRepositoryFactory();
     }