what is wrong with these codes ?

Question

whenever I want to create a multi user loginform this error pops up and gives me this notification
(System.NullReferenceException: 'Object variable or With block variable not set.' )

Imports System.Data.SqlClient
Public Class Form1
Private Sub Btnlogin_Click(sender As Object, e As EventArgs) Handles btnlogin.Click
Dim con As New SqlConnection("Data Source=Karwan;Initial Catalog=login;Integrated Security=True")
Dim txtuser As Object = Nothing
Dim txtpass As Object = Nothing
Dim cmbusertype As Object = Nothing
Dim cmd As SqlCommand = New SqlCommand(" select * from login where username='" & txtuser.Text & "'and password='" & txtpass.Text & "' and usertype= '" & cmbusertype.SelectedItem & "'", con)
Dim sda As SqlDataAdapter = New SqlDataAdapter(cmd)
Dim dt As DataTable = New DataTable()
sda.Fill(dt)
If (dt.Rows.Count > 0) Then
MessageBox.Show("چوونە ژوورەوە سەرکەوتوو بوو" + dt.Rows(0)(2))

        If (cmbusertype.selectedindex = 0) Then
            Dim a As New Admin
            a.Show()
            Me.Hide()
        Else
            Dim u As New User
            u.Show()
            Me.Hide()

        End If
    Else
        MessageBox.Show("زانیاریەکانت هەڵەیە")
    End If
End Sub

End Class

Answer 1

Probably you must remove these lines:

Dim txtuser As Object = Nothing
Dim txtpass As Object = Nothing
Dim cmbusertype As Object = Nothing

Assuming that the form contains such controls. Or use the name of the corresponding controls.

Note that it is possible to enter a special password without knowing the real correct password. To solve this vulnerability, consider "Parameterised queries".