Object reference is not set to the instance of an object

Public Class Maintainance
    Dim cn As New System.Data.SqlClient.SqlConnection
    Sub connect()
        cn = New System.Data.SqlClient.SqlConnection("Data Source=localhost;Initial Catalog=Fleet Maintainance;Integrated Security=True")
    End Sub
    Sub lockall()
        cBx1.Enabled = False
        cBx2.Enabled = False
        tBx1.Enabled = False
        tBx2.Enabled = False
        tBx3.Enabled = False
    End Sub
    Sub unlockall()
        cBx1.Enabled = True
        cBx2.Enabled = True
        tBx1.Enabled = True
        tBx2.Enabled = True
        tBx3.Enabled = True
    End Sub
    Sub setall()
        cBx1.Text = ""
        cBx2.Text = ""
        tBx1.Text = ""
        tBx2.Text = ""
        tBx3.Text = ""
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'Try
        If cBx1.Enabled = True Then
            Dim str As String
            str = "insert into AddMaintainanceTask (MainID, TypeID, PartCost, LaborCost, Total) values (" & (cBx1.SelectedValue.ToString()) & ", " & (cBx2.SelectedValue.ToString()) & ", " & CInt(tBx1.Text) & "," & CInt(tBx2.Text) & "," & CInt(tBx3.Text) & ")"
            Call connect()
            Dim cd As New System.Data.SqlClient.SqlCommand(str, cn)
            cd.Connection.Open()
            cd.ExecuteNonQuery()
            cd.Connection.Close()
            MsgBox(" New Task is added successfully ")
            Call lockall()
            Me.Close()
            Call IssueWorkOrder.listView2load()

        Else
            MsgBox(" Task is not added Try again ")

        End If
        'Catch ex As Exception
        'MsgBox(" Invalid Try agian ")
        'End Try
    End Sub
    Sub updatecombo1()
        Call connect()
        Dim cd As New System.Data.SqlClient.SqlCommand("SELECT [Name],[NameID] FROM [RepairName] order by [Name]", cn)
        Dim adp As New System.Data.SqlClient.SqlDataAdapter(cd)
        Dim ds As New DataSet
        adp.Fill(ds)
        'ComboBox2.Items.Clear()
        cBx1.DisplayMember = "Name"
        cBx1.ValueMember = "NameID"
        cBx1.DataSource = ds.Tables(0)
    End Sub
    Sub updatecombo2()
        Call connect()
        Dim cd As New System.Data.SqlClient.SqlCommand("SELECT [RepairType],[RepairTypeID] FROM [RepairType] order by [RepairType]", cn)
        Dim adp As New System.Data.SqlClient.SqlDataAdapter(cd)
        Dim ds As New DataSet
        adp.Fill(ds)
        'ComboBox2.Items.Clear()
        cBx2.DisplayMember = "RepairType"
        cBx2.ValueMember = "RepairTypeID"
        cBx2.DataSource = ds.Tables(0)
    End Sub
    Sub updatecombo3()
        Call connect()
        Dim cd As New System.Data.SqlClient.SqlCommand("SELECT [Service],[ServiceID] FROM [Service] order by [Service]", cn)
        Dim adp As New System.Data.SqlClient.SqlDataAdapter(cd)
        Dim ds As New DataSet
        adp.Fill(ds)
        'ComboBox2.Items.Clear()
        cBx1.DisplayMember = "Service"
        cBx1.ValueMember = "ServiceID"
        cBx1.DataSource = ds.Tables(0)
    End Sub
    Private Sub RadioButton1_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadioButton1.CheckedChanged
        RadioButton1.Text = "Preventive"
    End Sub

    Private Sub RadioButton2_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadioButton2.CheckedChanged
        RadioButton2.Text = "Repair"
    End Sub

    Private Sub RadioButton1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadioButton1.Click
        Call updatecombo3()
        Label3.Text = "Service"
        cBx2.Enabled = False

    End Sub

    Private Sub RadioButton2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadioButton2.Click
        Call updatecombo1()
        Call updatecombo2()
        Label3.Text = "Repair"
        cBx2.Enabled = True
    End Sub

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        AddService.Show()
        AddService.Visible = True
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Me.Close()
    End Sub

    Private Sub Maintainance_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    End Sub

    Private Sub tBx3_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles tBx3.TextChanged

    End Sub
End Class