Stuck with "Type of expression is ambiguous without more context" in prepare function

I'm new to coding Swift, so please excuse me if this error is a simple answer.. I am trying to transfer data to another viewcontroller, but struggling with "Type of expression is ambiguous without more context" error.

I made a @IBoutlet var mainTableView inside of the Viewcontroller right now.

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if let indexPath = mainTableView.indexPathForSelectedRow {
            habitSelected = habitlist[indexPath.row]
        }

Here's full code I made

https://i.sstatic.net/8mS4F.png

https://i.sstatic.net/Mzgwf.png

A alternative way to do that is to pass the row in the sender parameter

func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
    performSegue(withIdentifier: "unwindTableViewControiler", sender: indexPath.row)
}

and it's recommended to check the identifier of the segue before unwrapping the sender

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "unwindTableViewControiler" {
        let row = sender as! Int
        habitSelected = habitlist[row]
    }
}