Move down to only 1 abstract data type with a nully Frequency. Should aid in making the Expense page usable for all 3 data type.

lib/models/expense.dart

@@ -1,8 +1,8 @@
 // Local
-import '/models/tracked_type_recurring.dart';
+import '/models/tracked_item.dart';
 import '/models/frequency.dart';
 
-class Expense extends RecurringTrackedType {
+class Expense extends TrackedItem {
   static String amountText = "Cost";
 
   Expense({
@@ -29,7 +29,7 @@ class Expense extends RecurringTrackedType {
       'id': id,
       'name': name,
       'cost': amount,
-      'frequency': frequency.title,
+      'frequency': frequency!.title,
       'description': description,
     };
   }