Typo3 8.7.10 Flexform itemsProcFunc, custom controller function, repository is NULL

In my Typo3 plugins I want to give the admin the ability to select some items from a dropdown, the contents of which are stored in a "facilities" database, using the respository "FacilitiesRepository.php".

By default the system uses the UID as the value of each item. Therefore I want to use a custom function to ensure both the label and value of each item in the dropdown uses what I want.

I am using a flexform, and the "itemsProcFunc" tag to call my custom function. The custom function is in a controller called "FacilitiesController.php"

The controller uses inject to reference the respository, like so:

 /* @var \MyCompany\MyPlugin\Domain\Repository\FacilitiesRepository
 * @inject
 */
 protected $facilitiesRepository;

All functions within the controller successfully connect to the repository, so I know that it works.

I am trying to use the findAll() repository function.

My custom function is successfully called from the flexform, however whenever it references the facilitiesRepository it gives me the error: Call to a member function findAll() on null.

I tried replacing findAll with a test() function in the repository, but I still get : Call to a member function test() on null.

Null is always returned.

I have tried to inject the repo via other methods (I believe there are 3 ways) but the outcome is the same.

Here is my custom function:

public function findAllForFlexForm($config){ 

    $categories = $this->facilitiesRepository->findAll();  // <--here is the problem

    // create option list
    $optionList = array();

    foreach($categories as $key=>$item){
        $label = $item['id'];
        $value = $item['titel_de'];

        $optionList[] = array(0 => $label, 1 => $value);
    }

    // return config
    $config['items'] = array_merge($config['items'], $optionList);
    return $config;

}

Is there any reason why my facilitiesRepository is not recognised? If I ran this function from the FE it would return correctly.

In the FE your are in your Extbase context which is not the case in a hook in the backend. As extbase bootstrapping is rather expensive I propose you do a query using Doctrine DBAL and fetch the data directly.