Trying to make an excel reader in c# but it is throwing an error saying object reference not set to an instance of object

Question

private void cbosheet_SelectionChangeCommitted(object sender, EventArgs e)
    
{
     dt = dataTableCollection[cboSheet.SelectedIndex.ToString()];
        
     [var columnNames =  (from c in dt.Columns.Cast<DataColumn>()
                      
     select c.ColumnName).ToArray();]  
}            

System.NullReferenceException: 'Object reference not set to an instance of an object.'

dt was null.