ADO.NET DataGridView Update & Insert

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace ADO_Project
{
    public partial class myForm : Form
    {
        private string connectionString = "Server=VAKHO-PC; Database=TestDB; Integrated Security=true;";
        private SqlConnection connection = null;

        private SqlDataAdapter dataAdapter = null;
        private DataSet dataSet = null; // in-memory cache of data

        // create and open connection
        public myForm()
        {
            InitializeComponent();

            connection = new SqlConnection(this.connectionString);
            try
            {
                connection.Open();
            }
            catch (SqlException ex)
            {
                MessageBox.Show(ex.Message, "Connection Error!", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        // close the connection when we exit our form
        private void myForm_FormClosing(object sender, FormClosingEventArgs e)
        {
            try
            {
                connection.Close();
            }
            catch (SqlException ex)
            {
                MessageBox.Show(ex.ToString(), "Connection Error!", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        // checking connection state
        private void buttonState_Click(object sender, EventArgs e)
        {
            MessageBox.Show(connection.State.ToString(), "Connection State", MessageBoxButtons.OK);
        }

        private void myForm_Load(object sender, EventArgs e)
        {
            // setting select command inside the constructor
            dataAdapter = new SqlDataAdapter("SELECT * FROM dbo.Users", connection);

            // update command for adapter
            dataAdapter.UpdateCommand = new SqlCommand("UPDATE dbo.Users SET username = @username WHERE id = @id", connection);
            dataAdapter.UpdateCommand.Parameters.Add(new SqlParameter("@username", SqlDbType.NVarChar));
            dataAdapter.UpdateCommand.Parameters["@username"].SourceColumn = "username";
            dataAdapter.UpdateCommand.Parameters.Add(new SqlParameter("@id", SqlDbType.Int));
            dataAdapter.UpdateCommand.Parameters["@id"].SourceColumn = "id";

            dataSet = new DataSet("People"); // dataSet with dataSetName "People"
            dataAdapter.Fill(dataSet, "People"); // fill "People" table with rows (from start to end)

            dataGridView1.DataSource = dataSet.Tables["People"];

            // other stuff
            dataGridView1.Columns[0].ReadOnly = true;
        }

        private void dataGridView1_CellEnter(object sender, DataGridViewCellEventArgs e)
        {
            dataGridView1.BeginEdit(true);
        }

        private void dataGridView1_CellLeave(object sender, DataGridViewCellEventArgs e)
        {
            dataGridView1.EndEdit();
            dataAdapter.Update(dataSet, "People");
        }

        private void buttonAdd_Click(object sender, EventArgs e)
        {
            string username = textBox1.Text.Trim();
            if (username != "")
            {
                try
                {
                    // add new user
                    SqlCommand sqlCommand = new SqlCommand("INSERT INTO dbo.Users(username) VALUES ('" + textBox1.Text + "')", connection);
                    sqlCommand.ExecuteNonQuery();

                    dataAdapter.SelectCommand.ExecuteNonQuery(); // fetch new results from db
                    dataSet.Clear(); // clear old dataSet
                    dataAdapter.Fill(dataSet, "People"); // fill results into dataSet's "People" table

                    // refresh dataGridView1
                    dataGridView1.ClearSelection();
                    dataGridView1.DataSource = dataSet.Tables["People"];
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message, "Error while inserting new row!", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }
            }
            else
            {
                MessageBox.Show("Please enter at least one symbol!", "Empty Field Detected", MessageBoxButtons.OK, MessageBoxIcon.Warning);
            }
        }
    }
}