Return value of Beelab\UserBundle\Controller\UserController::getFilterFormName() must be of the type string, null returned

[lbm-services]

if you check for for:
if (null !== $filter = $this->getFilterFormName())

then declaration / type hinting should be:

protected function getFilterFormName(): ?string

Problem solved.

[garak]

What's the case when such return value is null?