DisMissState.build.<anonymous closure>.<anonymous closure>

showDialog can return null when the dialog is dismissed (e.g. user clicks outside the dialog) so you've to account for that by changing the type to:

final bool? res = await showDialog(/* your code */);

then in your logic below, you have to check for null:

if(res == null) {
 // handle dismiss
} else if (res == false) {
 // handle cancel
} else {
 // handle confirm/true
}

As @Er1 mentioned in the comment, you'll also need to pass true or false when popping the dialog:

Navigator.of(context).pop(true);
// or
Navigator.of(context).pop(false);

The above still applies since barrierDismissible is true by default.

final bool res = await showDialog(
                    context: context,
                    barrierDismissible: false,
                    builder: (BuildContext context) {
                      return AlertDialog(
                        content: Text('Are You Sure you want to delete'),
                        actions:<Widget> [
                          ElevatedButton(
                              onPressed: () {
                                Navigator.of(context).pop(false);
                              },
                              child: Text(
                                'cancel',

                              )),
                          ElevatedButton(
                              onPressed: () {
                                setState(() {
                                  genList.removeAt(index);
                                });
                                Navigator.of(context).pop(true);
                              },
                              child: Text(
                                'Delete',
                                style: TextStyle(color: Colors.red),
                              ))
                        ],
                      );
                    },
                  );

Add false and true to the pop() to make the showDialog return a boolean.

But keep in mind if you don't set barrierDismissible in showDialog to false you can get null returned if you tap outside of the dialog.

It is because when your calling the following code:

final bool res = await showDialog(...);

it will return a null value, see https://api.flutter.dev/flutter/material/showDialog.html

And, you also not giving any return value when clicking the button:

ElevatedButton(
     onPressed: () {
        Navigator.of(context).pop();
      },
      child: Text(
        'cancel',

      )),

You can't use Navigator.of(context).pop();. Instead, you need to use either:

Navigator.pop(context, false);

or

Navigator.pop(context, true);

Then, after that, you need to handle the nullable returned value from show dialog by giving a default value when null. Something like this:

bool res = await showDialog(...);
if(res == null) res = false;