Xcode – Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value in "self.tableView.realoadData()"

I made API Request and wanted to add it in Table View. Is realoadData() being called earlier than the loading of data?. There is an issue with tableView because the API call is working fine. Below is the code :

class ViewController: UIViewController {

var tableView : UITableView!
var listOfHolidays = [HolidayDetails](){
    didSet{
        DispatchQueue.main.async{
            self.tableView.reloadData()
            self.navigationItem.title = "(self.listOfHolidays.count) Holidays found"
        }
    }
}

calling Fetch Function in ViewDidLoad.

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view.
    DataFetch { (holidaydetails) in
        for items in holidaydetails{
            self.listOfHolidays.append(items)
            print(items)
        }
    
    }
}

Fetch Function for API Call.

func DataFetch(completionHandler : @escaping ([HolidayDetails]) -> Void){
    guard let url = URL(string: "https://holidayapi.com/v1/holidays?   pretty&country=IN&year=2020&key=457d9815-2e0a-4140-9515-0f6ef0d5f8b9") else{return(print("Error"))}
    
    let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
        
        guard let data = data else{ return print("Error")}
        do{
            let holidayData = try JSONDecoder().decode(Holidays.self, from: data)
            let holidayDetail = holidayData.holidays
            completionHandler(holidayDetail)
        }
        catch{
            let error = error
            print(error.localizedDescription)
        }
    }
    task.resume()
}
}

Extensions:

extension ViewController : UITableViewDataSource,UITableViewDelegate {

  func numberOfSections(in tableView: UITableView) -> Int {
      return 1
  }

  func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
      return listOfHolidays.count
  }

  func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
      let cell = tableView.dequeueReusableCell(withIdentifier: "cell",for: indexPath)
      let holidays = listOfHolidays[indexPath.row]
      cell.textLabel?.text = holidays.name
      cell.detailTextLabel?.text = holidays.date
      return cell
  }

}

I also have another file with Data Model :

import Foundation

struct Holidays : Codable {
   var holidays : [HolidayDetails]
}
struct HolidayDetails : Codable {
   var name : String
   var date : String
}