Kotlin would have definitely been a step up over Java purely for null safety.

In hindsight, RoR or Django would probably have been a better starting place considering their financial constraints (non-technical bootstrapping founders), but now that they are larger and more established Spring Boot seems a great framework given their current situation (the originally contracting code needed a major re-write anyway so I'm not sure that would have actually costed more time to go from RoR/Django -> Spring Boot)