[on_and_off]

Yes and no : I have been using kotlin for a while for Android. Currently our codebase is 75% kotlin.

Issues arose at the interface between java and kotlin. Unless there are @Nullable @NonNull annotations (and they need to be truthful), the kotlin compiler cannot know the nullability of something coming from a java method.

It can be pretty pernicious : if you use some java written libraries like Moshi (json parsing), it can also lead to crashes : IIRC if you declare a moshi generated property to be non null but it is absent from the json, it will generate an object with null, creating a crash.

Still, null is now an anecdotal issue in Kotlin. The Android framework team is working on annotating all their APIs with the corresponding nullability annotations and more and more JVM libs are also working on handling it gracefully.

It was never a huge issue to begin with even in java, just pretty cumbersome to have to add annotations everywhere and have to add some policies like 'no null collections, only empty' in a codebase to have sane handling.