Update progress bar with powershell and WPF error - wpf

Question

I'm tryig to do a simple GUI WPF who show a progress bar and copie file in a filetree.
I'm trying to update my progress bar with a Start-job but i get the error "[System.String] doesn't not contains method named "Invoke" and i didn't know how to update with multithreading.
This script take two parameters :
-File ( contain path to a file )
-Folder (it's an array of path)
Param(
[String]$file,
[String[]]$folder
)
Add-Type -AssemblyName PresentationFramework
Add-Type -AssemblyName System.Windows.Forms
[xml]$XML = Get-Content -Raw -Path ".\Logique\Copie.xaml"
$FormXML = (New-Object System.Xml.XmlNodeReader $XML)
$WindowCopie = [Windows.Markup.XamlReader]::Load($FormXML)
$pb_PROGRESS = $WindowCopie.FindName("pb_PROGRESS")
$lbl_MESSAGE = $WindowCopie.FindName("lbl_MESSAGE")
$btn_OK = $WindowCopie.FindName("btn_OK")
$btn_OK.Add_click({
$job = Start-Job -Name "Copie" -ScriptBlock $script -ArgumentList $pb_PROGRESS,$lbl_MESSAGE,$file,$folder
})
$script = {
Param($progressBar,$message,$file,$folder)
$progressBar.Dispatcher.Invoke([Action]{$progressBar.Maximum = $folder.Count},"Normal")
foreach ($val in $folder) {
$message.Dispatcher.Invoke([Action]{$message.Content = "Copie en cours $($progressBar.Value)/$($folder.count)"},"Normal")
$progressBar.Dispatcher.Invoke([Action]{$progressBar.Value++})
[System.IO.File]::SetAttributes($val, 'Normal')
Copy-Item -Path $file -Destination $val
}
}
$WindowCopie.ShowDialog()| Out-Null
and here my xaml file :
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="My WPF Interface" Height="150" Width="400">
<StackPanel>
<Label x:Name="lbl_MESSAGE" Content="My Message" HorizontalAlignment="Center" />
<ProgressBar x:Name="pb_PROGRESS" Minimum="0" Maximum="100" Width="360" Margin="17,10,16.057,10" Height="18" />
<Button x:Name="btn_OK" Content="OK" Width="100" Height="30" Margin="10,10,10,0" HorizontalAlignment="Center" />
</StackPanel>
</Window>
Thank's for the help
update