Swift Fatal Error optional nil when there is a value

Question

Fatal error: Unexpectedly found nil while unwrapping an Optional value

I am stuck on why I keep getting the error above

var linkFileSize   = ""   

class ViewController: NSViewController, URLSessionDelegate, URLSessionDownloadDelegate {

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {

        let filesize = Int64(linkFileSize)!
    
        let percentage = CGFloat(totalBytesWritten) / CGFloat(filesize)
    
        DispatchQueue.main.async{

            print("\(Int(percentage * 100))%")
        
        
        }
    
    }

    @IBAction func buttonClicked(_ sender: Any) {
        DownloadFile(downloadFile: downloadString)
    }

    func DownloadFile(downloadFile:String){

        let configuration = URLSessionConfiguration.default
    
        
        let urlSession = URLSession(configuration: configuration, delegate: self, delegateQueue: nil)
    
        guard let url = URL(string: downloadFile) else {return}
        
        let downloadTask = urlSession.downloadTask(with: url)
    
        downloadTask.resume()
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        downloadString = "My Download String from AWS"
        let fileSizeString = "set DLSize to do shell script(\"curl -s '\(downloadString)' | wc -c\") as text"
    
        linkFileSize = runAppleScriptString(aString: fileSizeString)

    }

    func runAppleScriptString(aString: String) -> String{

        var error: NSDictionary?
        if let scriptObject = NSAppleScript(source: aString) {
            let output: NSAppleEventDescriptor = scriptObject.executeAndReturnError(&error)
            print(output.stringValue ?? "")
            if (error != nil) {
                print("errorString: \(String(describing: error))")
                return ("errorString: \(String(describing: error))")
            }
            return output.stringValue ?? "\(String(describing: error))"
        
         }
    
         return ""
    }

}

I am trying to get the percentage of my download. I am able to get the size of the file as well as downlaod the file with no issues but when I try to get the percentage I run into an issue. When I print out the value of linkFileSize I get the correct size of the file. But then when I try to cast it to Int64 to the fileSize variable I keep getting a crash that the value is nil. How can the value be nil when I am able to print it out and see the value before right when the view loads but when I click the button and begin the DownloadFile function the value is suddenly nil?