Fix importing project into an unsaved project

[maiko3tattun]

Fixed an failure to get the relative path from an unsaved project to a wave part when trying to track import with wave part into an unsaved project. Ignore this exception if already got the relative path from the original project.

[stakira]

It does not make much sense to use the same relative path for the imported project and the unsaved project?

[maiko3tattun]

It is inevitable that AfterLoad will occur twice for the project we want to import and for the entire project after import. Therefore, I aim to avoid errors on the second AfterLoad.