System.NullReferenceException: 'Object reference not set to an instance of an object. How to fix

Question

I Run the code then it said System.NullReferenceException: 'Object reference not set to an instance of an object.' with any error. What should I do?

Imports MySql.Data.MySqlClient

Public Class Form1
    Dim gender As String


    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        openCon()
        MsgBox("Connected!")
        con.Close()

    End Sub

    Private Sub rbMale_CheckedChanged(sender As Object, e As EventArgs) Handles rbMale.CheckedChanged
        gender = "Male"
    End Sub

    Private Sub rbFemale_CheckedChanged(sender As Object, e As EventArgs) Handles rbFemale.CheckedChanged
        gender = "Female"
    End Sub

    Private Sub bntOkay_Click(sender As Object, e As EventArgs) Handles bntOkay.Click
        openCon()
        Try
            cmd.Connection = con
            cmd.CommandText = "INSERT INTO tbl_signup (`Full Name`, `Age`, `Gender`, `Birthday`, `Program`) Value('" & txtFullName.Text & "',
            '" & txtAge.Text & "', '" & gender & "', '" & dtpBirthday.Value.Date & "', '" & cbProgram.SelectedIndex & "') "
            cmd.ExecuteNonQuery()
            MsgBox("Registered Successfully!")
            con.Close()
            txtFullName.Clear()
            txtAge.Clear()
            rbMale.Checked = False
            rbFemale.Checked = False
            cbProgram.Text = " "

        Catch ex As Exception
            MsgBox(ex.ToString)

        End Try
    End Sub
End Class