bulk delete (userform submissions) results in JS error -> something with Symfony AbstractCache

[micschk]

Batch-deleting userform submissions:

Results in a console error:

Seems due to invalid JSON being returned:

{
  "isDestructive":true,
  "isError":false,
  "isWarning":true,
  "message":"Deleted 0 records.",
  "successClasses":[],
  "failedClasses":[],
  "records":{
    "success":[],
    "failed":[]
  }
}{
  "isDestructive":true,
  "isError":true,
  "isWarning":true,
  "message":"The \"Symfony\\Component\\Cache\\Simple\\AbstractCache\" class is deprecated since Symfony 4.3, use \"Symfony\\Component\\Cache\\Adapter\\AbstractAdapter\" and type-hint for \"Symfony\\Contracts\\Cache\\CacheInterface\" instead.",
  "successClasses":[],
  "failedClasses":[]
}