Fix bugs with snapshots: opening earliest snapshot, deleting latest

nummi/main/models.py

@@ -121,7 +121,7 @@ class Snapshot(models.Model):
             try:
                 _next = self.__class__.objects.get(previous=self)
             except self.__class__.DoesNotExist:
-                pass
+                super().delete(*args, **kwargs)
             else:
                 _next.previous = self.previous
                 super().delete(*args, **kwargs)

nummi/main/views.py

@@ -123,6 +123,9 @@ def snapshot(request, date=None):
         _transactions = None
     else:
         _snapshot = get_object_or_404(Snapshot, date=date)
+        if _snapshot.previous is None:
+            _transactions = None
+        else:
             _transactions = Transaction.objects.filter(
                 date__lt=_snapshot.date, date__gte=_snapshot.previous.date
             )