Typeerror: Return value must be of type string, none returned in getCount

Thx, can confirm this patch fixed it.

Imho this can be solved without changing the return type at Drupal\facets\Plugin\facets\facet_source\SearchApiDisplay::getCount().

The MR is working.
I'm not sure about removing the return type declaration, "?string" could be there. But since the return type declarations are not consistently used in the class and it's not used in the interface, removing is also an option that makes sense.
I mark the issue as RTBC.