quite a few UX improvements

frontend/lib/pages/new_trip_preferences.dart

@@ -1,5 +1,6 @@
 import 'package:anyway/layouts/scaffold.dart';
 import 'package:anyway/modules/new_trip_button.dart';
+import 'package:anyway/structs/landmark.dart';
 import 'package:anyway/structs/preferences.dart';
 import 'package:anyway/structs/trip.dart';
 import 'package:flutter/cupertino.dart';
@@ -20,6 +21,14 @@ class _NewTripPreferencesPageState extends State<NewTripPreferencesPage> with Sc
 
   @override
   Widget build(BuildContext context) {
+    // Ensure that the trip is "empty" save for the start landmark
+    // This is necessary because users can swipe back to this page even after the trip has been created
+    if (widget.trip.landmarks.length > 1) {
+      Landmark start = widget.trip.landmarks.first;
+      widget.trip.landmarks.clear();
+      widget.trip.addLandmark(start);
+    }
+
     return mainScaffold(
       context,
       child: Scaffold(