Error In Dataadapter.insert

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اثناء تنفيذ هذا الكود

public partial class test2 : Form
	{
		DataSet dataset = new DataSet();
		OleDbDataAdapter dataadapter = new OleDbDataAdapter();
		DataTable datatable;
		OleDbConnection con = new OleDbConnection();
		OleDbCommand cmd = new OleDbCommand();
		int aid;
		DataRowView dv;
		public test2()
		{
			InitializeComponent();
		}

		private void test2_Load(object sender, EventArgs e)
		{
			// TODO: This line of code loads data into the 'custDataSet.Company' table. You can move, or remove it, as needed.
			this.companyTableAdapter.Fill(this.custDataSet.Company);

		}

		private void button1_Click(object sender, EventArgs e)
		{
			dv = (DataRowView)comcombo.SelectedItem;
			aid = Convert.ToInt32(dv.Row["compid"]);
			con.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\\OccWork\\CustServ.mdb";
			cmd.Connection = con;
			cmd.CommandType = CommandType.Text;
			con.Open();

			dataadapter.InsertCommand.CommandText =
				"insert into service(no,date,trainer,summary,compid) " +
				"values(@x1,@x2,@x3,@x4,@x5)";

			cmd.Parameters.AddWithValue("@x1", Convert.ToInt32(notxt.Text));
			cmd.Parameters.AddWithValue("@x2", datetxt.Text);
			cmd.Parameters.AddWithValue("@x3", trainertxt.Text);
			cmd.Parameters.AddWithValue("@x4", summarytxt.Text);
			cmd.Parameters.AddWithValue("@x5", aid);
			dataadapter.Update(dataset, "service");
			dataset.AcceptChanges();
			con.Close();
		}

تظهر رسالة الخطأ التالية Object reference not set to an instance of an object.

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