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They didn't. A proper fix would require getting rid of null altogether in favor of ADTs or something similar. I work with C# daily and nulls can still slip through the cracks, although it's definitely better than @NotNull and friends.

I haven't worked with Kotlin in a while, but IIRC their non-nullable references actually do include runtime checks, so you cannot simply assign null to a non-nullable and have it pass at runtime like you can (easily) do in C#.