extension String {

    func index(at position: Int, from start: Index? = nil) -> Index? {
        let startingIndex = start ?? startIndex
        return index(startingIndex, offsetBy: position, limitedBy: endIndex)
    }

    func character(at position: Int) -> Character? {
        guard position >= 0, let indexPosition = index(at: position) else {
            return nil
        }
        return self[indexPosition]
    }
}

if let character = str.character(at: 3) {
    print("I found \(character)") // Нашел 1
}

if let character = str.character(at: 31) {
    print("I found \(character)") //Не найдет так как число слишком большое
}