Error while changing the selected item in listview. Help! ! ! ! !

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C#

.NET

ListView

I have a ListView named lvSelectCustomer.
I loaded some values in lvSelectCustomer. The lvSelectCustomer has 5 columns. Column[2] and Column[4] are hidden by set its Width into zero. Because i don't want to show those colums while runtime. All data are loaded successfully. No problem in loading data. The problem is...
I activate an event of lvSelectCustomer

private void lvSelectCustomer_ItemSelectionChanged(object sender, ListViewItemSelectionChangedEventArgs e)
{
string strSelectedCustomerIndex = lvSelectCustomer.FocusedItem.Index.ToString();
MessageBox.Show(strSelectedCustomerIndex);
}

Inside that event i created a string called strSelectedCustomerIndex . And set the focused item index to that string. While we select a row in lvSelectedCustomer at the 1st time the message box is coming and shows the index of that selected/focused row. Then after that, if i will select an another row from lvSelectedCustomer, an error is coming. Which is...
System.NullReferenceException: 'Object reference not set to an instance of an object.'
System.Windows.Forms.ListView.FocusedItem.get returned null.

I need the selected row's index while every row selection change. Please help me. I am not that much good in c#. Please help to sort it out. Thanks in advance.

What I have tried:

private void lvSelectCustomer_ItemSelectionChanged(object sender, ListViewItemSelectionChangedEventArgs e)
        {
            string strSelectedCustomerIndex = lvSelectCustomer.FocusedItem.Index.ToString();
            MessageBox.Show(strSelectedCustomerIndex);
        }