null safety

lib/pages/invoice/box_addition.dart

@@ -8,7 +8,7 @@ import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
 class BoxAddition extends StatefulWidget {
-  final Carton box;
+  final Carton? box;
   BoxAddition({this.box});
 
   @override
@@ -23,7 +23,7 @@ class _BoxAdditionState extends State<BoxAddition> {
   void initState() {
     super.initState();
     if (widget.box != null) {
-      _box = widget.box;
+      _box = widget.box!;
     }
   }
 
@@ -54,7 +54,7 @@ class _BoxAdditionState extends State<BoxAddition> {
                   child: Padding(
                 padding: const EdgeInsets.all(10.0),
                 child: ListView(children: <Widget>[
-                  DropdownButtonFormField(
+                  DropdownButtonFormField<Object>(
                     decoration: InputDecoration(
                         fillColor: Colors.white,
                         labelText: 'Box Number',