As you can see, by adding the “?” Character to the end of the type name, you are making the variable a nullable reference.

This will not create a compilation error. Kotlin also includes a specific Nullable type for each type that can be used to make a reference null. The code above fails to compile because Kotlin does not allow non-nullable reference types to be null. Non-nullable references are the first, and their type must be of a non-nullable type. There are two types of references in Kotlin: those that can't hold null and those that can. It happens when you try to access members that have a null reference. One of the most common challenges developers face for many programming languages, including Java, is the Null Pointer Exception. The type system in Kotlin was created with the Null pointer Exception in mind. If you've ever written code in Java or another language with the concept of null references, you've probably run across a NullPointerException. The program throws NullPointerExceptions at runtime, which might result in application failure or system crashes. Kotlin's type system aims to eradicate NullPointerException from the code.

Null safety in kotlin