null safety

lib/pages/invoice/editor/invoice_discount_list.dart

@@ -6,7 +6,7 @@ import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
 
 class InvoiceDiscountList extends StatelessWidget {
-  final List<Discount>? discounts;
+  final List<Discount?>? discounts;
 
   const InvoiceDiscountList({
     Key? key,
@@ -68,14 +68,14 @@ class InvoiceDiscountList extends StatelessWidget {
         onSelectChanged: (value) => Navigator.pop(context, c),
         cells: [
           MyDataCell(new Text(
-            c.code!,
+            c?.code ?? '',
             style: textStyle,
           )),
           MyDataCell(
             Row(
               mainAxisAlignment: MainAxisAlignment.end,
               children: [
-                Text(c.amount.toStringAsFixed(2), style: textStyle),
+                Text(c?.amount.toStringAsFixed(2) ?? "", style: textStyle),
               ],
             ),
           ),

lib/pages/invoice/editor/invoice_editor.dart

@@ -113,7 +113,7 @@ class _InvoiceEditorState extends State<InvoiceEditor> {
     });
   }
 
-  List<Discount> discounts = [];
+  List<Discount?> discounts = [];
   _loadDiscount() async {
     DiscountModel discountModel =
         Provider.of<DiscountModel>(context, listen: false);