Writing a ListBox value to another Form

I'am trying to write the current selected value in a listBox to another form. Here is the code so far:

public class MainForm : System.Windows.Forms.Form
      {
            private System.Windows.Forms.ListBox mainList;
            private UpdateStudent _updateForm;

.....

private void btnUpdate_Click(object sender, System.EventArgs e)
{
      this._updateForm = new UpdateStudent();
            _updateForm.ShowDialog();

      this._updateForm.stuName.Text = mainList.SelectedItem.ToString();      
}

It is trying to set the stuName text in the updateform to the currently selected Item in the mainList listbox. However, when I access the _updateForm, stuName is empty. Why is it not being populated with the text of the currently selected line in the listbox? Also, it compiles fine.

Thanks.