DialogManager.ShowDialogExternally() exception if no other window exists

[puchs]

Describe the bug
Idea: Display a styled MessageBox before the main window is created in case of an unrecoverable error (a RPC server cannot be started). I do not want to create a window just to tell the user the the app is going to shutdown now anyway. I think it is useful to be able to show a dialog without any other window from time to time.

var dialog = new CustomDialog() { Title = "Error" }; dialog = dialog.ShowDialogExternally();

in App.OnStartup should allow me to do just that as it creates a new window, but it will result in a

System.ArgumentException: 'Cannot set Owner property to itself.'

To Reproduce
Steps to reproduce the behavior:

1. get the repo https://github.com/puchs/MahAppsDialog
2. build it
3. run it

Expected behavior
I expect to see a dialog. I do not think another window is actually necessary for this to work. The same is true if a window has been created, but not yet shown. (This results in a different exception, but the reason is the same.)

Environment(please complete the following information):

• MahApps.Metro version 2.0.0-alpha0660
• OS: Win10 1909
• Visual Studio 2019 16.4.5
• .NET Core 3.1.101

Repo
https://github.com/puchs/MahAppsDialog

Additional context
I think the problem is located in src/MahApps.Metro/Controls/Dialogs/DialogManager.cs, line 597:

private static Window SetupExternalDialogWindow(BaseMetroDialog dialog, [CanBeNull] Window windowOwner = null)
{
var win = CreateExternalWindow();
win.Owner = windowOwner ?? Application.Current?.MainWindow;
...

as this will (as far as I can tell) set the owner of the window in any case. I believe that it should not set the property at all if windowOwner is null.