ShowModalMessageExternal problem with PowerShell

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## Describe the bug

I´m using Powershell with MahApps to display a GUI for the PS script.
After migrating from version 1.6.5 to 2.4.5, the display of dialogs no longer works. I get an exception with

"The calling thread cannot access this object because the object is owned by another thread."

## Steps to reproduce

A description of how to trigger this bug.

1. Klick button
2. Dialog should be displayed
3. Powershell crashes

Github repository: https://github.com/SirConfigMgr/InplaceNotification
UserInfo.ps1 in Line 459

## Expected behavior

Dialog should be displayed

## Actual behavior

Powershell crashes

## Environment

```
MahApps.Metro version: v2.4.5
Windows build number: Win10 1909 [Version 10.0.18363.836]
Visual Studio: 2019 16.6.0
Target Framework: .Net Core 3.1 (v4.5.2, v4.7.2)
```

Start with the external repository's UserInfo.ps1 at line 459 and reproduce the dialog flow described: click the button and observe the thread-access exception under MahApps.Metro 2.4.5. Compare the behavior with version 1.6.5 and identify the relevant dialog entry point; done means the PowerShell dialog displays without crashing.

Written by the indexing model from the issue text.