public static TransactionHistory CreateForm()   // create public static method with form type return
        {
            if (instance == null)
            {
                instance = new TransactionHistory();
            }

            return(instance);
        }
 private void TransactionHistoryForm_FormClosing(object sender, FormClosingEventArgs e)
 {
     instance = null; // explicitly set form instance to null as form closes
 }
 private void buttonTransactionHistory_Click(object sender, EventArgs e)
 {
     history = TransactionHistory.CreateForm();
     updateHistoryDisplay();
     history.Show();
 }