TypeError: strpos(): Argument #1 ($haystack) must be of type string, int given in strpos()

Problem/Motivation

As in the title, the strpos function is throwing the error because the given int key (of the form element) which is common in case
of the sequence form elements (keyed by int's - 0, 1, 2, 3...).

The current code looks like this:

      foreach ($plugins_config_form as $key => &$value) {
        if (is_array($value) && strpos($key, '#') === FALSE) {
          _add_ajax_listeners_to_plugin_inputs($value);
        }
      }

Steps to reproduce

Define any form elements in the plugin which will have some sequence. You can base it on the example schema:

        header:
          type: sequence
          label: 'Header'
          sequence:
            type: mapping
            mapping:
              html:
                type: string
                label: 'HTML'
              css:
                type: string
                label: 'CSS'
              type:
                type: string
                label: 'Type'

The error is thrown when trying to edit the given text format (ex. full HTML - /admin/config/content/formats/manage/full_html).
The plugin probably needs to be added first to the toolbar (so it could try to load the form)

Proposed resolution

There are few options, the simplest and still quite clean is to cast $key to string. I believe it might be even faster than adding additional checking for the int.
Still, we can do it these ways:
1.

      foreach ($plugins_config_form as $key => &$value) {
        if (is_array($value) && strpos((string) $key, '#') === FALSE) {
          _add_ajax_listeners_to_plugin_inputs($value);
        }
      }

2.

      foreach ($plugins_config_form as $key => &$value) {
        if (is_array($value) && !is_int($key) && strpos($key, '#') === FALSE) {
          _add_ajax_listeners_to_plugin_inputs($value);
        }
      }

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet