Flutter – set state function in the custom_dialog doesn't work

You are calling set state in the context of the dialog what you need to do is to call the set state in the context of the list view , modify your dialog to

import 'package:flutter/material.dart';
import 'package:untitled66/customers.dart';
import 'package:untitled66/main.dart';

class Dialog1 extends StatefulWidget {
    Dialog1({super.key, required this.refresh});
    final Function refresh;

  @override
  State<StatefulWidget> createState() {
    return dialog1State();
  }
}

class dialog1State extends State<Dialog1> {
  final TextEditingController customerNamecontr = TextEditingController();
  final TextEditingController itemNamecontr = TextEditingController();
  final TextEditingController pricecontr = TextEditingController();
  final TextEditingController profitcontr = TextEditingController();
  final TextEditingController periodcontr = TextEditingController();
  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: Text('Add customer'),
      content: Form(
          child: ListView(
        children: [
          Text("Customer's name:"),
          TextFormField(
            controller: customerNamecontr,
          ),
          Text("Item's name:"),
          TextFormField(
            controller: itemNamecontr,
          ),
          Text("Price:"),
          TextFormField(
            controller: pricecontr,
          ),
          Text("Profit:"),
          TextFormField(
            controller: profitcontr,
          ),
          Text("Period:"),
          TextFormField(
            controller: periodcontr,
          ),
        ],
      )),
      actions: [
        TextButton(
            onPressed: () {
              double? price = double.tryParse(pricecontr.text);
              int? profit = int.tryParse(profitcontr.text);
              int? period = int.tryParse(periodcontr.text);
              customersList.add(Customers(
                name: customerNamecontr.text,
                itemName: itemNamecontr.text,
                price: price!,
                profit: profit!,
                period: period!,
                key: UniqueKey(),
              ));
              widget.refresh();
              Navigator.of(context).pop();
            },
            child: Text('Add'))
      ],
    );
  }
}

and modify your main to

import 'package:flutter/material.dart';
import 'package:untitled66/custom_dialog.dart';
import 'package:untitled66/customers.dart';

List<Customers> customersList = [
  Customers(
    name: 'munsef',
    itemName: 'mobile',
    price: 100,
    profit: 30,
    period: 12,
    key: UniqueKey(),
  ),
];

void main() {
  runApp(const MaterialApp(
    home: MyApp(),
  ));
}

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() => MyAppState();
}

class MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
          floatingActionButton: FloatingActionButton(
              child: Icon(Icons.add),
              onPressed: () {
                showDialog(
                    context: context,
                    builder: (context) {
                      return Dialog1(refresh: () {
                    setState(() {});
                  });
                    });
              }),
          appBar: AppBar(
            title: Text('List of Items'),
            backgroundColor: Colors.lightBlue,
          ),
          body: ListView.builder(
              itemCount: customersList.length,
              itemBuilder: (element, index) {
                return InkWell(
                  child: Card(
                    child: customersList[index],
                  ),
                  onLongPress: () {
                    customersList.removeAt(index);
                    setState(() {});
                  },
                  
                );
              })),
    );
     }
     }

this will cause your child dialog to call the set state in ths context of its parent