Object reference not set to an instance of an object while running through cmd prompt

 public class IO
    {

        /// <summary>
        /// This method will initiate connection to open testdata document using OleDbConnection.
        /// </summary>
        /// <param name="excelPath">Path of the test data.</param>
        /// <returns>Returns the object initiated foe test data connection.</returns>
        private static OleDbConnection CreateExcelConnection(string excelPath)
        {
            // Provides connection information as a string.
            string oleDbCon = string.Format("Provider=Microsoft.ACE.OLEDB.12.0;Data Source={0}; Jet OLEDB:Engine Type=5;Extended Properties=Excel 12.0;", excelPath);

            // An instance of the OleDbConnection class is initiated.
            return new OleDbConnection(oleDbCon);

        } 

        /// <summary>
        /// This method will read excel data.
        /// </summary>
        /// <param name="excelPath">Path of the test data.</param>
        /// <param name="methodName">Test method name.</param>
        /// <returns>Return test data value in DataTable format.</returns>
        public static DataTable ReadExcel(
                string excelPath, 
                string methodName)
        {

            // 'using' block to dispose the object once the connection is no more needed.
            using (var excelconnection = CreateExcelConnection(excelPath))
            {
                try
                {
                    string columnName = "S No";  // Column name 'Sno' is assigned.
                    string excelSheetName = methodName.Split('_')[1];  // Test sheet name for each case has been taken from method name.
                    
                    // Opens the excel connection to read data required for the test method.
                    excelconnection.Open();

                    // Initiating the DataTable class.
                    DataTable DTExcelresult = new DataTable();

                    // Fetches the data from excel. 
                    OleDbDataAdapter DataAdapter = new OleDbDataAdapter(string.Format("Select * from [{0}$] where UCase([" + columnName + "]) IS NOT NULL ", excelSheetName), excelconnection);

                    // Stores the test data in a table format.
                    DataAdapter.Fill(DTExcelresult);
                    
                    // Ends the excel connection.
                    excelconnection.Close();

                    // Retun the data in table format.
                    return DTExcelresult;

                } 

                catch (Exception exception)
                {
                    // Write the exception to console.
                    Console.WriteLine("Exception", exception);

                    // Return null.
                    return null;

                } 

            } 

        } 

    }