Ссылка на объект не указывает на экземпляр объекта.

[Arte66]

Как пофиксить?

private void Update()
        {

            database.openConnection();

            for (int index = 0; index < dataGridView1.Rows.Count; index++)
            {
                var rowState = (RowState)dataGridView1.Rows[index].Cells[3].Value;

                if (rowState == RowState.Existed)
                    continue;

                if (rowState == RowState.Deleted)
                {
                    var id = Convert.ToInt32(dataGridView1.Rows[index].Cells[0].Value);
                    var deleteQuery = $"delete from sklad where id_user = {id}";

                    var command = new SqlCommand(deleteQuery, database.GetSqlConnection());
                    command.ExecuteNonQuery();
                }

                if (rowState == RowState.Modified)
                {

                    var id = dataGridView1.Rows[index].Cells[0].Value.ToString();
                    var den = dataGridView1.Rows[index].Cells[1].Value.ToString();
                    var name_rab = dataGridView1.Rows[index].Cells[2].Value.ToString();
                    var stavka = dataGridView1.Rows[index].Cells[3].Value.ToString();
                    

                    var changeQuery = $"update sklad set stavka = '{stavka}', den = '{den}', name_rab = '{name_rab}', where id_user = '{id}' ";

                    var command = new SqlCommand(changeQuery, database.GetSqlConnection());
                    command.ExecuteNonQuery();

                }

            }
            database.closeConnection();
        }

SELECT TOP (1000) [id_user]
      ,[den]
      ,[name_rab]
      ,[stavka]
  FROM [test].[dbo].[sklad]