IntelliJ says that I can’t assign `null` to `Triple?`

I have tried doing this code:

val positions = ArrayList<Triple<Int, Int, Int>?>() positions.add(null) 

IntelliJ says that there is an error at line 2 with the message: Null can not be a value of a non-null type Triple<Int, Int, Int>? , but the code compiles perfectly, is there a way to fix that?

EDIT: Updating the kotlin plugin helped