fix: avoid bug when checking current theme

this check is happening not only for classic views but API as well,
which doesn't really have a theme..  probably need a proper fix in
wuttaweb but this should be okay for now

tailbone/util.py

@@ -459,8 +459,8 @@ def should_use_oruga(request):
     supports (and therefore should use) Oruga + Vue 3 as opposed to
     the default of Buefy + Vue 2.
     """
-    theme = request.registry.settings['tailbone.theme']
-    if 'butterball' in theme:
+    theme = request.registry.settings.get('tailbone.theme')
+    if theme and 'butterball' in theme:
         return True
     return False