Handle nullables with errors, not panics

In several parts of the kotlin code, we are using `!!` to assert a value is not null. If the value is null, this will panic.

AC:
- Throw exceptions if value is null when it should be defined

Start by locating the Kotlin code paths that use `!!` and inspect what each nullable value represents before deciding which exception behavior is appropriate. Replace null assertions with exceptions where null indicates an invalid state, then verify that the affected paths no longer panic and that the acceptance criterion is met.

Written by the indexing model from the issue text.