Adding new slider slide fails

Problem/Motivation

I tried to add a new slider page through the settings and got this error:

TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function "addOne" not found or invalid function name in call_user_func_array() (line 105 of .../web/core/lib/Drupal/Core/Form/FormSubmitter.php).

I'm running a clean version of Drupal 11.1.