DBusPlatformSettings: NullReferenceException occurs if there is no DBusHelper.DefaultConnection

[pos777]

Describe the bug

The NullReferenceException occurs during Window creation

>	Avalonia.FreeDesktop.DBusPlatformSettings.TryGetThemeVariantAsync()
 	Avalonia.FreeDesktop.DBusPlatformSettings.TryGetInitialValuesAsync()
 	Avalonia.FreeDesktop.DBusPlatformSettings.OnRequestDefaultThemeVariant()
 	Avalonia.X11.X11Window.SetFrameThemeVariant(themeVariant)
 	Avalonia.Controls.TopLevel..ctor.AnonymousMethod__43_1(variant)
 	Avalonia.Controls.TopLevel.CreatePlatformImplBinding.__UpdatePlatformImpl|0()
 	Avalonia.Controls.TopLevel.CreatePlatformImplBinding<Avalonia.Styling.ThemeVariant>(property, onValue)
 	Avalonia.Controls.TopLevel.TopLevel(impl, dependencyResolver)
 	Avalonia.Controls.WindowBase.WindowBase(impl, dependencyResolver)
 	Avalonia.Controls.WindowBase.WindowBase(impl)
 	Avalonia.Controls.Window.Window(impl)
 	Avalonia.Controls.Window.Window()

Avalonia/src/Avalonia.FreeDesktop/DBusPlatformSettings.cs

Lines 45 to 49 in efaa926

	private async Task<PlatformThemeVariant?> TryGetThemeVariantAsync()
	{
	try
	{
	var version = await _settings!.GetVersionAsync();

It happens if the DBusPlatformSettings._settings field is not initialized due to the absence of DBusHelper.DefaultConnection:

Avalonia/src/Avalonia.FreeDesktop/DBusPlatformSettings.cs

Lines 19 to 24 in efaa926

	public DBusPlatformSettings()
	{
	if (DBusHelper.DefaultConnection is not { } conn)
	return;
	_settings = new Settings(conn, "org.freedesktop.portal.Desktop", "/org/freedesktop/portal/desktop");

To Reproduce

Tests in internal CI fail during migration to v12.1.0. Not sure how to reproduce this locally.

Expected behavior

No response

Avalonia version

12.1.0

OS

Linux

Additional context

Probably introduced in #16340