[C#] "Value cannot be null" Help

I have a method that runs if either Form1 loads (the main form) or if Form2 is closed. The method populates a combobox from a text file. When the form loads, the combobox populates just fine. But, when Form2 is closed I get a "Value cannot be null. Parameter name: item". I'm lost as to why it works when the form loads, but not when the 2nd form closes.

The code for the method in question (which is located in Form1):

public void PrinterDD()
    {
        string sprinters = @"pathtofile";
        using (StreamReader sr = new StreamReader(sprinters))
        {
            foreach (object item in sprinters)
            {
                string strPrnItem = sr.ReadLine();
                printer.Items.Add(strPrnItem);     //error occurs here
            }
        }
    }

The code in Form2:

private void accept_Click(object sender, EventArgs e)
    {
        this.AcceptButton = accept;
        if (selectedprinters != null && selectedprinters.Items != null && selectedprinters.Items.Count > 0)
        {
            string txtfile=@"pathtofile";
            File.Delete(txtfile);
            using (StreamWriter writer = new StreamWriter(File.OpenWrite(txtfile)))
            {
                foreach (object item in selectedprinters.Items)
                {
                    writer.WriteLine(item.ToString());
                }
            }
        }
        Voicemails.PrinterDD();
        this.Close();
    }