How do I solve nullreferenceexception for below code. Help me guys I have been stuck for days! ! ! !

    public partial class Admin_Control : Form
    {
        Form1.User aa;
        public string sa;
        private Form1.User ua;

        public Admin_Control(Form1.User s1)
        {
            InitializeComponent();
            this.ua = ua;
            aa = s1;
        }

        private void button7_Click(object sender, EventArgs e)
        {
            Main mn = new Main(ua);
            mn.Show();
            this.Hide();

        }

        private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {

            if (e.ColumnIndex == 1)
            {

                OleDbConnection con = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=D:\\IT Inventory management\\IT Inventory Database.accdb");
                OleDbCommand cmd = con.CreateCommand();
                cmd.CommandText = "UPDATE Login SET UserRole ='Admin'";
                con.Open();
                cmd.ExecuteNonQuery();
                MessageBox.Show("User Role Updated");
            }
            else
            {
                MessageBox.Show("Changes Failed");
            }
              
        }
        private void show()
        {
            DataTable dt = new DataTable();
            OleDbConnection con = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=D:\\IT Inventory management\\IT Inventory Database.accdb");
            OleDbCommand cmd = con.CreateCommand();
            cmd.CommandText = "SELECT * FROM Login";
            con.Open();
            OleDbDataReader adr= cmd.ExecuteReader();
            dt.Load(adr);
            dataGridView1.DataSource = dt;
            //dataGridView1.DataBind();
         DataGridViewButtonColumn btn = new DataGridViewButtonColumn();
         btn.HeaderText = "Change User Role";
         btn.Name = "btn_change";
         btn.Text = "Change";
         btn.UseColumnTextForButtonValue = true;
         dataGridView1.Columns.Add(btn);    
           
        }
        
        private void button1_Click(object sender, EventArgs e)
        {
            dataGridView1.Visible = true;
            show();
           
        }
        private void uname()
        {
            string admi = "";
            OleDbConnection con = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=D:\\IT Inventory management\\IT Inventory Database.accdb");
            OleDbCommand cmd = con.CreateCommand();
            cmd.CommandText = "SELECT UserRole FROM Login WHERE Username='" + sa + "' ";
            con.Open();
            OleDbDataReader adm;
            adm = cmd.ExecuteReader();
        }

        private void Admin_Control_Load(object sender, EventArgs e)
        {

            sa = aa.name; **
            lbl_name.Text = aa.name;

            uname(); 
        }

        private void button3_Click(object sender, EventArgs e)
        {
            New_User nu = new New_User();
            nu.Show();
            this.Close();
        }

       
            
        }

        
}

This is the code and I get Error on the code marked with **