changed up the way that ingredients can be added

lib/models/ingredient.dart

@@ -2,12 +2,12 @@ import 'unit.dart';
 
 class Ingredient {
   final String title;
-  List<Unit> possibleUnits = [];
-  List<String> preferredBrands = [];
+  List<Unit> possibleUnits;
+  List<String> preferredBrands;
 
   Ingredient({
     required this.title,
     required this.possibleUnits,
-    required this.preferredBrands,
+    this.preferredBrands = const [],
   });
 }

lib/models/unit.dart

@@ -9,8 +9,6 @@ class Unit {
 enum System { metric, imperial, neutral }
 
 enum UnitType {
-  metric,
-  imperial,
   fluid,
   weight,
   count,