Swift: App crashes when accessing NULL value from SQLite.swift

i just started learning Swift4. I have read all the suggestions for stack overflow duplicate questions, but I could not solve or understand my problem.

I am fetching Data from a SQLite.swift database:

ViewController.swift:

import Foundation
import SQLite

// Load DB with SQLite.swift
let path = Bundle.main.path(forResource: "myDataBaseFile", ofType: "sqlite")!
let db = try! Connection(path, readonly: true)

// SQLite.swift define table
let table = Table("table")

// SQLite.swift define columns 
let id = Expression<Int>("id")
let medium = Expression<String>("medium")

// Function to get query result
func queryForMedium(idForQuery: Int) -> String? {
    let media = try? db.scalar(table.select(medium).filter(id == idForQuery))
    return media
}

// ViewDidLoad
override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    // Value for ID = 2 access a SQL Null value
    anOptionalString = queryForMedium(idForQuery: 2)

    if anOptionalString != nil {
        print("Contains a value!")
    } else {
        print("Doesn’t contain a value.")
    }

}

Result:

• ID = 1 contains "Test" -> App runs: Console Output:
  Contains a value!
• ID = 2 contains NULL -> CRASH: Console Output:
  SQLite was compiled with optimization - stepping may behave oddly; variables may not be available.

Note: I have tried to disable Optimazion Level (Build Settings), the result is the same.

I would be grateful for help or solutions!