how to count duplicated rows in datagridview and consider duplicates as single row and add count as quantity

Question

       string cid = dgv.Rows[0].Cells[0].Value.ToString();

       for (int i =1; i<dgv.Rows.Count;i++)
        {
            string x = dgv.Rows[i].Cells[0].Value.ToString();

            if(x == cid)
            {
                dgv.Rows[i].Cells[4].Value =+ 1;
                dgv.Rows.RemoveAt(i);
            }
            else
            {
                cid= dgv.Rows[i].Cells[0].Value.ToString();
            }
        }

cid << is checking id

cell[4]<< is quantity (count)default is 1

dgv.Rows[i].Cells[0].Value.ToString();

this line shows,

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

error when executed.