Fatal error (memory exhaustion) when visiting nodes/users that do not exists.
Create test site (for example simpletest.me)
Install module
Create second language
Place advanced language block
Visit node or user that does not exist
There seems to be an issue with "Url::createFromRequest" in LanguageSelectorBlock.php when the request is a 404 page.
In the core language switcher there is a check for front page/ 404 routematch, perhaps something like this can be used.
see "core\modules\language\src\Plugin\Block\LanguageBlock.php" line 92
Fixed
1.3
Code