Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'sliderwidgetValidatePositiveNumber' not found or invalid function name

Problem/Motivation

When configuring the widget display settings, the following error message pops up:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'sliderwidgetValidatePositiveNumber' not found or invalid function name.
This is because the callback defined in #element_validate for the step-setting needs to be a multi-dimensional array.

Steps to reproduce

* configure a number field to use the SliderWidget
* update and save the form display settings

Remaining tasks

* a merge request has been created