Fix: Fixed crash that would occur when using an invalid background image

Author: yaira2

src/Files.App/ViewModels/MainPageViewModel.cs

@@ -89,9 +89,10 @@ public float AppThemeBackgroundImageOpacity
 			=> AppearanceSettingsService.AppThemeBackgroundImageOpacity;
 
 		public ImageSource? AppThemeBackgroundImageSource =>
-			string.IsNullOrEmpty(AppearanceSettingsService.AppThemeBackgroundImageSource)
-				? null
-				: new BitmapImage(new Uri(AppearanceSettingsService.AppThemeBackgroundImageSource, UriKind.RelativeOrAbsolute));
+			!string.IsNullOrEmpty(AppearanceSettingsService.AppThemeBackgroundImageSource) &&
+				Uri.TryCreate(AppearanceSettingsService.AppThemeBackgroundImageSource, UriKind.RelativeOrAbsolute, out Uri? validUri)
+					? new BitmapImage(validUri)
+					: null;
 
 		public VerticalAlignment AppThemeBackgroundImageVerticalAlignment
 			=> AppearanceSettingsService.AppThemeBackgroundImageVerticalAlignment;