How to do I add new column after endling of loop inside Gridview?

I want to add a new column after the end of the loop inside Gridview.
Right now in my code I add the append functionality in floatingActionButton but I want to add this append functionality in the column after every end of the column loop.

Like this:- need to add a new column after end of loop

Please see this image for a better understanding.

I hope you understand what I want to say.

Right now my output is like this:- see there is floatingActionButton where i add append functionality to add a new column

Here is my code:-

import 'package:flutter/material.dart';
import 'package:mindmatch/utils/widget_functions.dart';
import 'package:mindmatch/custom/BorderIcon.dart';
import 'package:mindmatch/screens/Relation.dart';
import 'package:flutter_svg/flutter_svg.dart';

void main() {
 runApp(new MaterialApp(
  home: new Photos(),
));
}

class Photos extends StatefulWidget {
 var usrid;

 Photos({Key? key, @required this.usrid}) : super(key: key);

 @override
 _Photos createState() => _Photos();
}

class _Photos extends State<Photos>{

int counter = 0;
//List<Widget> _list = List<Widget>();
List<Widget> _list = <Widget> [];


@override
 void initState() {

 for (int i = 0; i < 2; i++) {
   Widget child = _newItem(i);
   _list.add(child);
 };
}

void on_Clicked() {
 Widget child = _newItem(counter);
 setState(
      () => _list.add(child),
 );
}

Widget _newItem(int i) {
 Key key = new Key('item_${i}');
 Column child = Column(
    key: key,
    children: [
      Stack(
          children:  [
            Card(
              elevation: 0,
              shape: RoundedRectangleBorder(
                side: BorderSide(
                  color: Color(0xffa1a1a1),
                ),
                borderRadius: BorderRadius.all(Radius.circular(12)),
              ),
              child: SizedBox(
                //width: 300,
                height: 100,
                child: Center(child:
                Icon(
                  Icons.image,
                  color: Color(0xffcccccc),
                  size: 60,
                ),

                ),
              ),
            ),
            Positioned(
              top: 9,
              right: 9,
              child: InkWell(
                onTap: () => _removeItem(i),
                child: SvgPicture.asset(
                  width: 20,
                  'assets/images/close.svg',
                  height: 20,
                ),
              ),
            )
          ]
      ),
    ]
);
counter++;
return child;
}

void _removeItem(int i) {
 print("====remove $i");

 print('===Removing $i');
 setState(() => _list.removeAt(i));
}


@override
Widget build(BuildContext context) {

return new Scaffold(
  appBar: new AppBar(
    title: new Text('Photo'),
    backgroundColor: Colors.deepPurpleAccent,
  ),

  floatingActionButton: new FloatingActionButton(onPressed: on_Clicked, child: new 
  Icon(Icons.add),),

  body: new Container(
      padding: new EdgeInsets.all(32.0),
      child: Column(
        children: [
          Expanded(
              child: GridView(
                //padding: const EdgeInsets.all(8.0),
                gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
                  crossAxisCount: 3,
                  crossAxisSpacing: 10.0,
                  mainAxisSpacing: 15,
                  //childAspectRatio: 2/1,
                ),
                  children: [
                    itemBuilder: List.generate(_list.length, (index) {
                          //generating tiles with people from list
                          return _newItem(index);
                        },
                  ]
                  return Column(

                  )
                  ),
              )
          )
        ],
      ),
  ),
);
}


}

Please help how I add a new Column after the end of the loop on that column i add onTap property where i add append functionality to add new column when click.

Here is the code of the new column with add icon

InkWell(
    onTap: (){},
        child: Column(
              children: [
                Stack(
                    children: const [
                      Card(
                        elevation: 0,
                        color: Color(0xff8f9df2),
                        shape: RoundedRectangleBorder(
                          side: BorderSide(
                            color: Color(0xff8f9df2),
                          ),
                          borderRadius: BorderRadius.all(Radius.circular(12)),
                        ),
                        child: SizedBox(
                          //width: 300,
                          height: 100,
                          child: Center(child:
                          Icon(
                            Icons.add,
                            color: Colors.white,
                            size: 80.0,
                          ),

                          ),
                        ),

                      )
                    ]
                ),
            ]
         ),
    )

>Solution :

You can extend item-count (item Length) by one and use last index to show add widget.

children: List.generate(itemLength + 1,
        (index) => index == itemLength ? Text("new Item") : _newItem(index)),