单元测试ASP.net Page_Load

我正在使用Visual Studio Unit测试框架构建工作.我想创建一个单元测试来检查网页的元素及其值.

我知道硒及其在单元测试中的能力.

这是测试WebPageControl.ascx.vb的网页：

Public Class WebPageControl
        Protected Sub Page_Load(ByVal sender As Object,ByVal e As System.EventArgs) Handles Me.Load
            TextBox.Visible = False
        End Sub
End Class

这是单元测试WebPageControlTest.vb：

Public Class WebPageControlTest
    Public Sub PageLoadTest()

        Dim target As WebPageControl_Accessor = New WebPageControl_Accessor() 
        Assert.IsFAlse(target.TextBox.Visible)
    End Sub
End Class

我这样做后仍然出错

Test method RechargeTest.WebPageControlTest.PageLoadTest threw exception: System.NullReferenceException: Object reference not set to an instance of an object.