Rather than trying to make use of a specific Overworld level from the started server for CasterTomeRegistry and doing so only for a RegistryAccess, it would make more sense for the latter to be retrieved directly from the started server itself independently of any level.

In situations such as when using the test server provided by NeoForge and ModDevGradle for unit testing, this prevents any such test runs from crashing and hanging due to the fact that the test server doesn't have any levels to speak of and returns null when trying to retrieve them, which up until now was never checked or accounted for.