Return value must be of type string, null returned

1. +++ b/src/Form/AuditFilesManagedNotUsedForm.php
   @@ -139,7 +139,10 @@ final class AuditFilesManagedNotUsedForm extends FormBase implements AuditFilesA
   +    if (count($rows) > 0)  {
   

   This is an irrelevant change.

   In any case instead $rows should be put at the top of the function instead of doing this empty check.

   Create a new issue.

2. +++ b/src/Services/AuditFilesConfig.php
   @@ -65,7 +66,8 @@ final class AuditFilesConfig implements AuditFilesConfigInterface {
   +    return $auditfiles_report_options_date_format ? $auditfiles_report_options_date_format : '';
   

   This value should never be null, if the module is installed fresh this wont happen. Theres a chance that evolution of the module fixed a possible null.

   Instead of the proposed change, I'd accept a hook update to fix this value so it is forced to a string.

I can verify that I had the same problem but a simple re-install of the module fixes it.
My auditfiles.settings only had one line and throwing NULL errors but reinstalling the module added more settings.

I checked in my system didn't encountered any issue in /admin/reports/auditfiles/mergefilereferences