Why Does my Form Try to Dispose When Accessing ListBox.SelectedItem.ToString()?

Question

I'm developing a small POS for a university project. I have a form which acts as a POS main window, with a datagrid and so on. Also, I have one form who is the Sensitive search or Incremental search, and I want that form to, select one item in a listbox and return it to the main window. Now I have a property in the main which gets that item as a string, and when the user clicks the OK button on the search form, I want to set that property on the main window.

Everything works great except one thing: when I try to access listBox_Codigo.SelectedItem.ToString(); the app tries to dispose and closes all windows... Does anybody know why?

I just need the selected string in that listbox and set it to the main window like this:

var Principal = (PDQ.Cajero)this.ParentForm;
                Principal.CodigoInsertado = listBox_Codigo.SelectedItem.ToString();
                this.DialogResult = DialogResult.OK;
                this.Close();

where PDQ.Cajero is the main form, which calls this form.

UPDATE: I just finished debugging it, and right after the program gets to listBox_Codigo.SelectedItem.ToString(); the program jumps to Dispose().

UPDATE 2 This is my complete method:

private void button1_Click(object sender, EventArgs e)
    {
        if (listBox_Codigo.SelectedItem == null)
        {
            if (MessageBox.Show(this, "No se puede ingresar un producto sin seleccionarlo.\n ¿Desea intentarlo de nuevo, o Salir?", "Error", MessageBoxButtons.RetryCancel, MessageBoxIcon.Exclamation) == DialogResult.Cancel)
            {
                DialogResult = DialogResult.Cancel;
                this.Close();
            }
        }
        else
        {
            var Principal = (PDQ.Cajero)this.ParentForm;
            Principal.CodigoInsertado = listBox_Codigo.SelectedItem.ToString();
            this.DialogResult = DialogResult.OK;
            this.Close();

        }
    }

So the problem is not if the value is null...

Answer 1

There likely is no SelectedItem (meaning that the value of the property is null). In this case your code is throwing a NullReferenceException, since you can't call a function on a null reference. Because you aren't catching it, the application is catching it at a higher level an attempting to exit. This is what's calling your Dispose method.