在我的PowerShell脚本中导致空值表达式的原因是什么？

Question

我正在尝试使用PowerShell脚本打开IE并登录到Intranet站点。该脚本将打开IE并加载页面，但我收到以下错误：

You cannot call a method on a null-valued expression.
At D:\Admin Programs\Bartender\Print Portal Logon Scripts\DahliaPPLogonv2.ps1:9 char:1
+ $usernamefield = $ie.Document.getElementByID('UserName')
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

The property 'value' cannot be found on this object. Verify that the property
exists and can be set.
At D:\Admin Programs\Bartender\Print Portal Logon Scripts\DahliaPPLogonv2.ps1:10 char:1
+ $usernamefield.value = 'jbob'
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : PropertyNotFound

每个变量的错误重复，我无法弄清楚原因。完整的脚本如下。

# Create an Internet Explorer object
$ie = New-Object -ComObject 'InternetExplorer.Application'
$ie.Visible= $true # Make it visible
# Open website
$ie.Navigate("http://bws01/BarTender/Account/Login") 
#Wait till webpage is loaded
while ($ie.Busy -eq $true) {Start-Sleep -Seconds 5;}
# Feed credentials to the input fields on the web page
$usernamefield = $ie.Document.getElementByID('UserName')
$usernamefield.value = 'jbob'
$passwordfield = $ie.Document.getElementByID('Password')
$passwordfield.value = "123456"
$chk = $ie.Document.getElementById("RememberMe").Checked = $true 
$submit = $ie.Document.getElementById("LogonPageSubmitButton").Click()