How can I put variables in Form1 class listBox from another class?

Question

How can I put variables into Form1 class listBox from another class?

I declare variables in Form2 window to third class named Product by properties. After this the second window should close and send those variables to MainWindow (Form1) listBox. I tryed to make instruction putting variables from Form2 window to MainWindow , but I am getting a NullException error. Now code looks like this:

Class Form1 :

public partial class Form1 : Form
{
    Form2 fileAdd;
    Product product;

    public Form1()
    {
        InitializeComponent();
        fileAdd = new Form2();
        product = new Product();
    }

    public void actualizationButton_Click(object sender, EventArgs e)
    {
        if (product.Name == statusList.Text) // if product is in the listBox
        {
            // the instruction that will be performed if the given product is on the listBox
            for (int i = product.Quantity; i < int.Parse(quantityTextBox.Text); i++)
            {
                product.Quantity++;
            }
        }
        else
        {
            fileAdd.Show(); // launches the window to adding a new item

            if (product.Name.Length != 0 && product.Code.Length != 0 &&
                product.Reference.Length != 0 && product.Manufacturer.Length != 0)
            {
                statusList.Items.Add(product.Name + "\t" + product.Reference + "\t" +
                product.Manufacturer + "\t" + product.Quantity);

                product.Name = "";
                product.Code = "";
                product.Reference = "";
                product.Manufacturer = "";
                product.Quantity = 0;
                // how to execute the instruction to transfer variables 
                //to listBox Form1 after closing Form2 window?
            }
            else
            {
                MessageBox.Show("Not included item!");
            }
        }
    }
}

Class Form2 :

public partial class Form2 : Form
{
    Product product;

    public Form2()
    {
        InitializeComponent();
        product = new Product();
    }

    private void fileProductAddButton_Click(object sender, EventArgs e)
    {
        if (product.Name.Length != 0 && product.Reference.Length != 0)
        {
            product.Name = fileNameTextBox.Text;
            product.Code = fileCodeTextBox.Text;
            product.Reference = fileReferenceTexBox.Text;
            product.Manufacturer = fileManufacturerTextBox.Text;

            MessageBox.Show("Added " + product.Name + " with reference " + product.Reference +
                "\nManufacturer " + product.Manufacturer + "\nQuantity " + product.Quantity);

            this.Close();
        }
        else
        {
            MessageBox.Show("Fill in all fields!");
        }
    }
}

Class Product :

class Product
{
    public string Name { get; set; } = "N/A";
    public string Code { get; set; } = "N/A";
    public string Reference { get; set; } = "N/A";
    public string Manufacturer { get; set; } = "N/A";
    public int Quantity { get; set; } = 0;
}

Answer 1

Pushing data from one form to another should be a basic thing, there are several methods to do that. First: implement a proper constructor which accepts a Product object. Second option is to create a public Property which can be filled with the data after creation of the form. And third, you can have a static field, holind gyour data. Every option is up to you and your design.

Answer 2

TinoZ suggestions are fine, but seeing you are a beginner, I'll add a little bit of code to illustrate the concepts.
Clearly your problem is that Form1 and Form2 have completely separate instances of a product. Whatever you do with the product in Form2 has no effect in Form1.

Your main form creates an instance of Form2 - here you can take the new instance of the Product and pass it to 'Form2' as a dependency, like below:

        Form2 fileAdd;
        Product product;

        public Form1()
        {
            InitializeComponent();
            product = new Product();
            fileAdd = new Form2(product);
            
        }

Then, your Form2 constructor needs to understand this parameter and assign it to it's field:

        Product product;

        public Form2(Product product) //this is the product from main form
        {
            InitializeComponent();
            this.product = product; //here you are making Form2 work on the same product that Form1 has.
        }

Lastly, please tidy up your code, remove the 'noise' and if you are getting any errors (like null reference), post them as well.