System.InvalidOperationException: "Fill: Свойство SelectCommand.Connection не инициализировано."

using MySql.Data.MySqlClient;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
 
 
namespace Bus_Avtopark
{
    public partial class LoginForm : Form
 
    {
        DataBase db = new DataBase();
 
        public LoginForm()
        {
            InitializeComponent();
 
            StartPosition = FormStartPosition.CenterScreen;
 
            this.passField.AutoSize = false;
            this.passField.Size = new Size(this.passField.Size.Width, 64);
        }
 
        private void closeButton_Click(object sender, EventArgs e)
        {
            this.Close();
        }
 
        private void closeButton_MouseEnter(object sender, EventArgs e)
        {
            closeButton.ForeColor = Color.Red;
        }
 
        private void closeButton_MouseLeave(object sender, EventArgs e)
        {
            closeButton.ForeColor = Color.Green;
        }
        Point lastPoint;
        private void panel1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                this.Left += e.X - lastPoint.X;
                this.Top += e.Y - lastPoint.Y;
            }
        }
 
        private void panel1_MouseDown(object sender, MouseEventArgs e)
        {
            lastPoint = new Point(e.X, e.Y);
        }
 
        private void buttonUser_Click(object sender, EventArgs e)
        {
            var loginuser = loginField.Text;
            var passuser = passField.Text;
 
            MySqlDataAdapter adapter = new MySqlDataAdapter();
            DataTable table = new DataTable();
 
            
            MySqlCommand command = new MySqlCommand
                            ("SELECT * FROM 'register' WHERE login_user = ul AND password_user = up"+ db.getConnection());
            command.Parameters.Add("ul", MySqlDbType.VarChar).Value = loginuser;
            command.Parameters.Add("up", MySqlDbType.VarChar).Value = passuser;
 
            adapter.SelectCommand = command;
            adapter.Fill(table);
 
 
            if (table.Rows.Count == 1)
                MessageBox.Show("Yes");
            else
                MessageBox.Show("No");
        }
      }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.SqlClient;
 
 
namespace Bus_Avtopark
{
    internal class DataBase
    {
 
        SqlConnection sqlConnection = new SqlConnection
            (@"Data Source = DKU - 1083\SQLEXPRESS;Initial Catalog = Avtopark; Integrated Security = True");
 
 
 
        public void openConnection()
        {
            if (sqlConnection.State == System.Data.ConnectionState.Closed)
            {
                sqlConnection.Open();
            }
        }
 
        public void closeConnection()
        {
            if (sqlConnection.State == System.Data.ConnectionState.Open)
            {
                sqlConnection.Close();
            }
        }
 
        public SqlConnection getConnection()
        {
            return sqlConnection;
 
        }
 
    }
}

("SELECT * FROM 'register' WHERE login_user = ul AND password_user = up"+ db.getConnection());

            MySqlCommand command = new MySqlCommand
                            ("SELECT * FROM 'register' WHERE login_user = @ul AND password_user = @up", db.getConnection());
            command.Parameters.Add("@ul", MySqlDbType.VarChar).Value = loginuser;
            command.Parameters.Add("@up", MySqlDbType.VarChar).Value = passuser;