Fatal error when saving Solo theme settings

On the latest Solo theme release (version 1.0.24), saving the theme settings — even without making changes — causes a fatal error in _solo_theme_settings_submit().

I have just updated the Solo theme to version 1.0.24.
When I go to the theme settings page and click “Save configuration” — even without changing anything — I get this error:

Error: Call to undefined method Drupal\Core\Asset\LibraryDiscoveryCollector::clearCachedDefinitions() in _solo_theme_settings_submit() (line 231 of /themes/contrib/solo/theme-settings.php)

Steps to reproduce:
1. Go to Appearance → Settings for the theme.
2. Click “Save configuration”, even without making any changes.

Result:
The page shows the fatal error and the settings are not saved.

Request:
I’m a beginner with Drupal and not sure what this error means. Could you please advise how I can fix this?

Thank you.