Void error after assigning an UIImage from the web in Swift

Question

When trying to generate a random image from the web, I'm getting a fatal error because nil was found when returning the self.imageVar in the generateImage() function after resuming the data task.

import UIKit

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {
    @IBOutlet weak var tableView: UITableView!
    var imageVar: UIImage?
    
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        tableView.dataSource = self
        tableView.delegate = self
        generateImage()
       
    }
    
    func generateImage() -> UIImage {
        let url = URL(string: "https://picsum.photos/200/300")
        let task = URLSession.shared.dataTask(with: url!) {
            (data, response, error) in
            
            if let data = data {
               DispatchQueue.main.async() {
                   self.imageVar = UIImage(data: data)
               }
                
            } else {
                print("Error")
            }
        }
        task.resume()
        return self.imageVar!
    }

    @IBAction func reloadImage(_ sender: Any) {
        generateImage()
    }
    
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 4
    }
    
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "cell")!
        
        cell.imageView?.image = imageVar
        return cell
    }
}

How can I fix this?