If optimization fails (e.g. narrow band) if the project is saved, it write outs particle filenames that don't exist and becomes unreadable

[akenmorris]

Steps to reproduce:

1. Run an optimization that fails due to narrow band issues
2. Save project
3. Reload project

The project will fail to load:

To fix it, one must open the project file and remove the particle files.

When optimization fails, the particle filenames should be empty when the project saves.