title: How to deal with type mismatch- String and String?
body: <p>I'm getting a type mismatch when I want to perform an assignment.
  The required value is String but I have got String?. My question is: how should I deal with it, via toString method or rather by adding non-null asserted(!!) call. I'm wondering which of them is better for safety or maybe performance. The question is rather about best practices for a language.</p>