带有Webbrowser控件的VB.Net/Word Access HtmlElement

Question

我目前正在使用Word插件。从功能区启动Web浏览器控件。如果后者已经打开，我想修改文本框的内容并模拟按钮单击。问题是，当我尝试访问文本框时出现NullPointerException。我似乎有一些类似的问题，但是没有一种解决方案对我有用。这是我的代码：

Private Sub btn_Click(sender As Object, e As RibbonControlEventArgs) Handles btn.Click

    Dim frame As New frm_browser()
    Dim search As String
    Dim url As String
    Dim WordApp As Microsoft.Office.Interop.Word.Application = Globals.ThisAddIn.Application
    Dim doc As HtmlDocument = frame.browser.Document
    Dim htmlSearchField As HtmlElement
    Dim htmlSearchButton As HtmlElement

    search = WordApp.Selection.Text

    If IsFormOpen(browser.GetType()) = False Then
        frame.Show()
        url = "http://example.com?search=" + search
        frame.browser.Navigate(url)
    Else
        'Trigger the error and the htmlElement returns null
        htmlSearchField = doc.GetElementById("tb_id")
        htmlSearchField.SetAttribute("value", search)
        htmlSearchButton = doc.GetElementById("btn_id")
        htmlSearchButton.InvokeMember("click")
    End If
End Sub

这是我尝试访问的html部分：

<input id="tb_id" name="xxx" type="text" value="">
<div id="btn_id">Contains an image that simulate a search button</div>

任何提示都会受到赞赏。谢谢您的帮助！