How to fix the error (optional swift)?

Question

Swift

About a week ago I started learning swift and came across a bug that I can't solve. Topic: optional data types. In general, I more or less figured out everything to everyone, but I can’t do the unraping of the String value?
Conditions:

let number2 :  String? = "78hyf9"

if number2 != nil {
    sum = sum + Int(number2!)!
} else {
    print("number2 is nil")
}

Error
error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).
Fatal error: Unexpectedly found nil while unwrapping an Optional value

Answer 1

First you need to learn how to formulate your question, and only then look at such a funny moment. If you've entered the if block, then you're already sure that your optional string contains a value. That's just where did you get the idea that your krakozyabra in the line will be reduced to a number? Obviously it will not come down and it will not come down. The expression Int(number2!) returns nil
Plus, you should be reading about Optional Binding already.