error: Object reference not set to an instance of an object.--in Create user wizard step

 hi
i am using a custom create user wizard in my project in this i wrote a block of code in my CreateUserWizard1_CreatedUser event 
the code is as follows 
 Dim step1 As CreateUserWizardStep
        Dim UserNameTextBox As TextBox = DirectCast(step1.ContentTemplateContainer.FindControl("UserName"), TextBox)
        Dim DataSource As SqlDataSource = DirectCast(step1.ContentTemplateContainer.FindControl("InsertExtraInfo"), SqlDataSource)

        Dim User As MembershipUser = Membership.GetUser(UserNameTextBox.Text)
        Dim UserGUID As Object = User.ProviderUserKey

        DataSource.InsertParameters.Add("UserId", UserGUID.ToString())
        DataSource.Inser ...