Fix migration; fix invoice saving

nummi/main/models.py

@@ -279,11 +279,12 @@ class Invoice(CustomModel):
         Transaction, on_delete=models.CASCADE, editable=False
     )
 
-    def save(self):
+    def save(self, *args, **kwargs):
         if Invoice.objects.filter(id=self.id).exists():
             _prever = Invoice.objects.get(id=self.id)
             if _prever.file and _prever.file != self.file:
                 pathlib.Path(_prever.file.path).unlink(missing_ok=True)
+        super().save(*args, **kwargs)
 
     def __str__(self):
         if hasattr(self, "transaction"):