Runtime exception with c++ filesystem

1. load sfizz LV2 in ardour
2. load a sfz file of your choice
3. delete the sfz file
4. it crashes shortly after with a runtime error

```
** (ardour-6.0.pre1.333:117023): ERROR **: 20:43:25.815: 
unhandled exception (type std::exception) in signal handler:
what: Aucun fichier ou dossier de ce type: '/home/jpc/desktop/alex1.sfz'
```