PowerShell GUI - Function within a Form not working - forms

Question

currently i am working on an little PowerShell GUI Porject.
I have a Form within this Form there's a Button to call another Form that provides an Listbox with some items in it.
So far so good - i'm getting the results as expected.
Now i mark one of the items - click on OK - then it should do something.
And here is my Problem, it does nothing :D No Error, no warning, just nothing.
If i test with write-host "$($listBox.SelectedItem)" - i get the selected Item as i expect it.
But if you want to use it with get-adfsrelyingpartytrust -Name $listBox.SelectedItem - nothing happens.
If i execute the function without the rest (so that only the listbox appears) everything works as intended O_o.
Code:
function single_rpt{ $RPTForm = New-Object System.Windows.Forms.Form
$RPTForm.ClientSize = '400,200'
$RPTForm.Text = "Choose"
$RPTForm.BackColor = "#b2b2b2"
$RPTForm.AutoSize = $false
$RPTForm.StartPosition = 'CenterScreen'
$RPTForm.ControlBox = $true
$rptButton = New-Object System.Windows.Forms.Button
$rptButton.BackColor = "#ffffff"
$rptButton.Text = "OK"
$rptButton.Width = 90
$rptButton.Height = 30
$rptButton.Location = New-Object System.Drawing.Point(20,125)
$rptButton.Font = 'Microsoft Sans Serif,10'
$rptButton.ForeColor = "#000000"
$rptButton.UseVisualStyleBackColor = $true
$rptButton.DialogResult = 1
# Test List Box
$listBox = New-Object System.Windows.Forms.ListBox
$listBox.Location = New-Object System.Drawing.Point(20,50)
$listBox.Size = New-Object System.Drawing.Size(350,200)
$listBox.Height = 80
$RPTForm.Controls.AddRange(#($listBox,$rptButton))
$listboxArray = #()
$ADFS_RPTS = Get-AdfsRelyingPartyTrust
Foreach($rpt in $ADFS_RPTS)
{
#$Object = Add-Member -TypeName NoteProperty -Name Service -Value $service.Name
$listboxArray += $rpt.Name
}
$listBox.Items.AddRange($listboxArray)
[void]$RPTForm.ShowDialog()
if($RPTForm.DialogResult -eq "OK")
{
Get-AdfsReylingPartyTrus -Name $listBox.SelectedItem
}
}
# Init PowerShell Gui
Add-Type -AssemblyName System.Windows.Forms
# Create Form with size, title and background color, etc.
$TESTForm = New-Object System.Windows.Forms.Form
$TESTForm.ClientSize = '250,300'
$TESTForm.Text = "TEST"
$TESTForm.BackColor = "#ffffff"
$TESTForm.AutoSize = $false
$TESTForm.StartPosition = 'CenterScreen'
$TESTForm.ControlBox = $true
$RPTSingle = New-Object System.Windows.Forms.Button
$RPTSingle.BackColor = "#ffffff"
$RPTSingle.Text = "RPTSingle"
$RPTSingle.Width = 90
$RPTSingle.Height = 30
$RPTSingle.Location = New-Object System.Drawing.Point(75,125)
$RPTSingle.Font = 'Microsoft Sans Serif,10'
$RPTSingle.ForeColor = "#000000"
$RPTSingle.UseVisualStyleBackColor = $true
$RPTSingle.Add_Click({single_rpt})
# Add a Cancel Button
$CancelButton = New-Object System.Windows.Forms.Button
$CancelButton.BackColor = "#ffffff"
$CancelButton.Text = "Cancel"
$CancelButton.Width = 90
$CancelButton.Height = 30
$CancelButton.Location = New-Object System.Drawing.Point(75,250)
$CancelButton.Font = 'Microsoft Sans Serif,10'
$CancelButton.ForeColor = "#000000"
$CancelButton.UseVisualStyleBackColor = $true
$CancelButton.DialogResult = 2
$TESTForm.Controls.AddRange(#($RPTSingle,$CancelButton))
# Display the form
[void]$TESTForm.ShowDialog()
#------------------------------------#