loading screen stuck fix (#15011)

- small change to force update of the front end after a corrupt file is being opened
- in case of an error, re-setting the `ShowStartPage` will trigger the update to the front end and remove the loading screen

src/DynamoCoreWpf/ViewModels/Core/DynamoViewModel.cs

@@ -1842,6 +1842,8 @@ private void Open(object parameters)
                         commandString,
                         MessageBoxButton.OK,
                         MessageBoxImage.Error);
+
+                    this.ShowStartPage = true;  // Triggers start page change to notify the frontend
                 }
                 else
                 {