PHPUnit: PHP Fatal error: Call to a member function find() on a non-object

I have a function in my HandlerClass that works fine:

/**
     * @param $entity
     * @return null|object
     */
    public function findEntityById($id)
    {
        if($id)
        {
            $entity = $this->repository->find($id);
            if (empty($entity)) {
                return false;
            }
            return $entity;
        }
        return false;
    }

I wrote a test for a function:getCheckedList(), which uses this function:findEntityById(). I mocked the function:findEntityById(), so that it should return false.

$reviewHandler = $this->getMock(
            'My\Bundle\Handler\RevHandler',
            array('saveEntity'),
            array(
                $this->entityManager,
                'My\Bundle\Entity\Rev',
                $this->guzzleClient,
                $this->serializer,
                $this->apiSettings
            )
        );

        $reviewHandler->expects($this->once())
            ->method('findEntityById')
            ->will($this->returnValue(false));

        $result = $reviewHandler->getCheckedList(22);

        $this->assertArrayHasKey(0,$result);
        $this->assertEquals(22,$result[0]);

But then I get an error after my test:report:phpunit

PHP Fatal error:  Call to a member function find() on a non-object on line 152

And this is this line in my function:findEntityById() which is mocked and which throws an error in my test:

$entity = $this->repository->find($id);

In the __construct form my RevHandler I call the parent::__construct:

public function __construct($entityManager, $entityClass, $guzzleClient, $serializer, $apiSettings)
    {
        parent::__construct($entityManager, $entityClass, $guzzleClient, $serializer, $apiSettings);
    }

In my parent::__construct it looks like this:

public function __construct(EntityManager $entityManager, $entityClass, $guzzleClient, $serializer, $apiSettings)
    {
        $this->entityManager = $entityManager;
        $this->entityClass = $entityClass;
        $this->repository = $this->entityManager->getRepository($this->entityClass);
        $this->guzzleClient = $guzzleClient;
        $this->serializer = $serializer;
        $this->apiSettings = $apiSettings;
    }

The code without testing works fine, but I don't know what can be wrong when I'm testing. Any idea? THANKS!!!