Dispatch Queue - main and in background situation with var

Hello!

I want to verify a property of a tagBarItem but the navigation controller needs to be accessed on the main queue.

However, I have a blocking call that I want to be processed in the background.

The problem is that in line 15 when I access the value of petitionString it is empty.

What am I doing wrong?

performSelector(inBackground: #selector(fetchJSON), with: nil)

@objc func fetchJSON() {
        
        var petitionURLString: String = ""
        
        DispatchQueue.main.async {
            [weak self] in
            if self?.navigationController?.tabBarItem.tag == 0 {
                petitionURLString = "https://www.hackingwithswift.com/samples/petitions-1.json"
            }
            else {
                petitionURLString = "https://www.hackingwithswift.com/samples/petitions-2.json"
            }
        }
        
        if let url = URL(string: petitionURLString) {
            if let data = try? Data(contentsOf: url) { //Este método é chamado de blocking call bloqueia a execução de código até todos os dados estarem descarregados.
                //É seguro fazer parsing dos dados mas como pode dar erro usamos try?
                parseData(json: data)
                return
            }
        }

        showError()
    }