Testing singleton initialization and throwing appropriate exceptions - powershell

Question

I am continuing to try to grok a singleton implementation with PowerShell classes, and having issues now with handling exceptions.
My test singleton needs to be initialized the first time with a list of files to process, and all following references should have no arguments, since I never want to reprocess files, just reference what was processed initially. To that end I have this...
class TestClass {
# Properties
[collections.arrayList]$Files = [collections.arrayList]::New()
static [TestClass] $instance = $null
# Constructor
TestClass([string[]]$filePaths){
foreach ($path in $filePaths) {
$this.Files.Add("$($path): Processed")
}
}
[void] ListFiles() {
foreach ($processedFile in $this.Files) {
Write-Host "$processedFile!"
}
}
# Singleton Methods
static [TestClass] GetInstance() {
if ([TestClass]::Instance -ne $null) {
$caller = (Get-PSCallStack)[-1]
throw "Px Tools Exception: Singleton must be initialized ($caller)"
}
return [TestClass]::Instance
}
static [TestClass] GetInstance([string[]]$filePaths) {
if ([TestClass]::Instance -eq $null) {
[TestClass]::Instance = [TestClass]::New($filePaths)
} else {
$caller = (Get-PSCallStack)[-1]
throw "Px Tools Exception: Singleton cannot be reinitialized ($caller)"
}
return [TestClass]::Instance
}
}
A properly done first initialization works, i.e.
$firstInstance = [TestClass]::GetInstance($unprocessedFiles)
$firstInstance.ListFiles()
will initialize the singleton and list the files. So the conditional if ([TestClass]::Instance -eq $null) works.
Also, if I try to reinitialize like this
$firstInstance = [TestClass]::GetInstance($unprocessedFiles)
$secondInstance = [TestClass]::GetInstance($unprocessedFiles)
I get the appropriate
Exception: Singleton cannot be reinitialized
However, if I make the first reference without the files to process, like this...
$firstInstance = [TestClass]::GetInstance()
I get no error. So, why does if ([TestClass]::Instance -eq $null) seem to work correctly but if ([TestClass]::Instance -ne $null) does not? And, is there a better way to address this? I thought perhaps I could test the $Files property, either for $null or a Count of 0, but I can't reference a non static property from a static method, so barring the kludgy approach of a static $Initialized property, what is the solution?
EDIT: Duh. Hopefully this helps someone else. Don't test for the value, test for existence, like this.
if (-not [TestClass]::Instance) {