Fatal error after saving theme settings.

[msnassar]

After updating the theme to recent release. We encounter fatal error after saving the theme settings.
Steps to reproduce:

• On the theme settings page, untick "Card image hidden on mobile".
• When submitting the form, we encounter the following fatal error.

The website encountered an unexpected error. Please try again later.

TypeError: Drupal\oe_bootstrap_theme\BackwardCompatibility::getSetting(): Return value must be of type bool, int returned in Drupal\oe_bootstrap_theme\BackwardCompatibility::getSetting() (line 31 of themes/contrib/oe_bootstrap_theme/src/BackwardCompatibility.php).

oe_bootstrap_theme_form_system_theme_settings_alter() (Line: 378)
Drupal\system\Form\ThemeSettingsForm->buildForm()
call_user_func_array() (Line: 536)
Drupal\Core\Form\FormBuilder->retrieveForm() (Line: 283)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
Drupal\Core\Controller\FormController->getContentResult()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 592)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 48)
Drupal\ucpkn_core\StackMiddleware\RedirectMiddleware->handle() (Line: 58)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 704)
Drupal\Core\DrupalKernel->handle() (Line: 19)

[brummbar]

@msnassar , your child theme should extend oe_bootstrap_theme settings schema, as it's done here:
https://github.com/openeuropa/oe_whitelabel/blob/1.x/config/schema/oe_whitelabel.schema.yml
I tried on oe_whitelabel and I cannot reproduce the bug.