Flutter BottomNavigationBar: why selected item shifts other icons?

I'm new to flutter and I created a BottomNavigationBar and add into it some BottomNavigationBarItem that contains flutter icons.

The problem is that, when I select one item of my BottomNavigationBar, this item shifts other icons.

this is a screenshot of my app:

is there a way to block this shift ?

my code:

import 'package:flutter/material.dart';

class Navbar extends StatefulWidget {
  const Navbar({ Key key }) : super(key: key);

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

class _NavbarState extends State<Navbar> {

int index = 0;

@override
Widget build(BuildContext context) {
  return BottomNavigationBar(
    iconSize: 30,
    showSelectedLabels: false,
    showUnselectedLabels: false,
    selectedItemColor: Colors.blueGrey,
    unselectedItemColor: Colors.black,
    currentIndex: index,
    onTap: (int selectedIndex) {
      setState(() {
        index = selectedIndex;
      });
    },
    items: [
      BottomNavigationBarItem(
        icon: new Icon(
          Icons.home,
        ),
        title: new Text('Home'),
      ),
      BottomNavigationBarItem(
        icon: new Icon(
          Icons.search,
        ),
        title: new Text('Search'),
      ),
      BottomNavigationBarItem(
        icon: Icon(
          Icons.add,
        ),
        title: Text('Add')
      ),
      BottomNavigationBarItem(
        icon: Icon(
          Icons.favorite,
        ),
        title: Text('Add')
      ),
      BottomNavigationBarItem(
        icon: Icon(
          Icons.account_circle,
        ),
        title: Text('Account')
      )
    ],
  );
}
}