When renaming siteDefinitions it can easily happen, that you have an extends of an non existing siteDefinition. Reproduce:
Rename the siteDefinition demo-project to demo-project-test and restart
The system is not reachable anymore.
(the siteDefinition demo-project-de is extending the demo-project which doesn't exist anymore).

The class should be changed, that it uses in such a case the default siteDefintion as a fallback.