Fix fetching of messages in frontend

aleksis/core/frontend/components/app/App.vue

@@ -283,7 +283,9 @@ export default {
     },
     $route: {
       handler(newRoute) {
-        this.$apollo.queries.messages.refetch();
+        if (this.$apollo.queries.messages) {
+          this.$apollo.queries.messages.refetch();
+        }
       },
       immediate: true,
     },