Default CSS selector not read from configuration

Problem/Motivation

The default CSS selector to initiate the tabled library is coming back null from the configuration, so users must explicitly set the value on their configuration page for the library to work.

Steps to reproduce

1. Remove any values from /admin/config/content/ckeditor-responsive-table, or do a fresh Drupal install
2. Open src/Form/CkeditorResponsiveTableForm.php
3. Add kint($default_value); after the default value check in buildForm (line 34 as of this writing)
4. Reload the page
5. Notice value is null

Proposed resolution

The value should be pulled from the default_table_selector key in config/install/ckeditor_responsive_table.settings.yml. The issue is probably some small error in configuration fetching. This needs to be identified and corrected.