Error: Call to a member function getString() on null in alerts_views_pre_render() (line 24 of modules/contrib/alerts/alerts.module).

The configuration for that vocabulary is provided by the module, and should have been imported when the module was installed. Did you encounter any errors during install?

Actually, looking at the module's code I believe I see the problem. For the import of the configuration to work as expected the taxonomy module needs to be installed, but it isn't listed as a requirement in alerts.info.yml. I will update to fix that, but that won't help a site where this module is already installed.

If you're still having this problem I would suggest manually importing the taxonomy.vocabulary.alert_severity.yml configuration file.

Automatically closed - issue fixed for 2 weeks with no activity.