DomainThemeSwitchConfigForm error messages

Problem/Motivation

1. After activating the module, there are several error messages on the configure page:

Error message
Warning: Trying to access array offset on false in Drupal\domain_theme_switch\Form\DomainThemeSwitchConfigForm->buildForm() (line 146 of modules/contrib/domain_theme_switch/src/Form/DomainThemeSwitchConfigForm.php).

Error message
Warning: Trying to access array offset on false in Drupal\domain_theme_switch\Form\DomainThemeSwitchConfigForm->buildForm() (line 147 of modules/contrib/domain_theme_switch/src/Form/DomainThemeSwitchConfigForm.php).

Error message
Warning: Undefined array key "admin" in Drupal\domain_theme_switch\Form\DomainThemeSwitchConfigForm->buildForm() (line 147 of modules/contrib/domain_theme_switch/src/Form/DomainThemeSwitchConfigForm.php).

Error message
Warning: Trying to access array offset on false in Drupal\domain_theme_switch\Form\DomainThemeSwitchConfigForm->buildForm() (line 146 of modules/contrib/domain_theme_switch/src/Form/DomainThemeSwitchConfigForm.php).

Error message
Warning: Trying to access array offset on false in Drupal\domain_theme_switch\Form\DomainThemeSwitchConfigForm->buildForm() (line 147 of modules/contrib/domain_theme_switch/src/Form/DomainThemeSwitchConfigForm.php).

2. Additionally after enabling the theme override, there are other errmor messages:

Error message
Warning: Trying to access array offset on false in Drupal\domain_theme_switch\Form\DomainThemeSwitchConfigForm->submitForm() (line 235 of modules/contrib/domain_theme_switch/src/Form/DomainThemeSwitchConfigForm.php).

Error message
Warning: Trying to access array offset on false in Drupal\domain_theme_switch\Form\DomainThemeSwitchConfigForm->submitForm() (line 236 of modules/contrib/domain_theme_switch/src/Form/DomainThemeSwitchConfigForm.php).

Error message
Deprecated function: Automatic conversion of false to array is deprecated in Drupal\domain_theme_switch\Form\DomainThemeSwitchConfigForm->submitForm() (line 247 of modules/contrib/domain_theme_switch/src/Form/DomainThemeSwitchConfigForm.php).

Steps to reproduce

1. Install the module composer require 'drupal/domain_theme_switch:^3.0@alpha'
2. Active it
3. Go to the "Configure" page while having active domain records

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3a. Enable a theme override and more errors will appear.