Selection changed textbox value C#

If datagrid no have rows in my textbox have values from last selected values...

Need help IF DATAGRID NO HAVE VALUES TEXTBOX EMPTY

Thank you  

private void testDataGridView_SelectionChanged(object sender, EventArgs e)  
       {  

           if (testDataGridView.SelectedRows.Count > 0)   
           {  

           string test = testDataGridView.SelectedRows[0].Cells["columnname"].Value.ToString();  

testTextBox.Text = test;  

            }  
        }