Error when saving user configuration

Problem/Motivation

I installed this module in Drupal 8.9.16 version and when I set the Notify Settings for a user, when I try to save the configurations, I get the error message:

The website encountered an unexpected error. Please try again later.
Error: Call to a member function delete() on null in Drupal\notify\Form\UserSettings->submitForm() (line 231 of modules/contrib/notify/src/Form/UserSettings.php).

Steps to reproduce

1) Install the module in a Drupal 8 site version
2) Go to People -> select a user -> notify settings
3) Put your configurations
4) Click in Save Configuration

Proposed resolution

I notice that $db_connection is not declared in submitForm function from UserSettings.php file. Declare this variable there.

Remaining tasks

User interface changes

API changes

Data model changes