Why is the object created in the class destroyed?

Question

OOP

In general, I am writing in the form of c # and I ran into the problem of the program crashing, now I will try to explain.
in the main class of the form I create a link to an object on another form, later I fill in the datagrid in this other form with data from the first

Result r1 = new Result();

public void test()
{
    for (int i = 0; i < Convert.ToInt32(timeCreditText.Text.ToString()); i++)
    {
        try
        {
            r1.dataGridView1.Rows.Add();
            r1.dataGridView1.Rows[i].Cells[0].Value = (i + 1).ToString();
        }
        catch(Exception e)
        {
            MessageBox.Show("error");
        }
    }
    r1.Show();
}

and the problem is that when I close window 2 of the form and click again to open it and fill it with data, I get an exception that System.ObjectDisposedException: "Access to the disposed object is not possible."
I hide that other form like this: MainForm.r1.Hide(); and it is not clear why the reference to the object is destroyed.
BUT if you create a new object each time you click, then everything is OK, and I used this, but it’s not clear why the created object of another form in the main class of the main form is destroyed when that other form is hidden.