cleanup code

lib/pages/rates/custom_editor.dart

@@ -35,8 +35,7 @@ class _CustomEditorState extends State<CustomEditor> {
       _custom = widget.custom!;
       _productController.text = _custom.name ?? "";
       _feeController.text = _custom.customDutyFee.toStringAsFixed(2);
-      _shipmentRateController.text =
-          _custom.rate == null ? "" : _custom.rate.toStringAsFixed(2);
+      _shipmentRateController.text = _custom.rate.toStringAsFixed(2);
     } else {
       _isNew = true;
     }

lib/pages/rates/custom_list.dart

@@ -83,6 +83,7 @@ class _CustomListState extends State<CustomList> {
                       child: _row(
                           custom.name??"",
                           "Custom Fee \$ " + custom.customDutyFee.toStringAsFixed(2),
+                          // ignore: unnecessary_null_comparison
                           custom.rate == null
                               ? ""
                               : "Shipment rate \$ " +

lib/pages/rates/shipment_rates.dart

@@ -200,7 +200,7 @@ class _ShipmentRatesState extends State<ShipmentRates> {
   }
 
   List<Widget> getDiscountWidget(List<DiscountByWeight> discounts) {
-    if (discounts == null) return [];
+    if (discounts.isEmpty) return [];
     return discounts.map((d) {
       return Container(
         child: _row("${d.weight.toStringAsFixed(2)} lb",

lib/pages/rates/shipment_rates_calculate.dart

@@ -67,7 +67,7 @@ class _ShipmentRatesCalState extends State<ShipmentRatesCal> {
     setState(() {
       _deliveryFee =
           effectiveWeight > rate.freeDeliveryWeight ? 0 : rate.deliveryFee;
-      _amount = amount == null ? 0 : amount + _deliveryFee;
+      _amount = amount + _deliveryFee;
       _shipmentWeight = shipmentWeight.toDouble();
     });
   }
@@ -109,7 +109,7 @@ class _ShipmentRatesCalState extends State<ShipmentRatesCal> {
     );
 
     final shipmentWeightBox = DisplayText(
-      text: _shipmentWeight != null ? _shipmentWeight.toStringAsFixed(2) : "0",
+      text: _shipmentWeight.toStringAsFixed(2),
       labelTextKey: "box.shipment_weight",
       iconData: MaterialCommunityIcons.weight,
     );

lib/pages/rates/shipment_rates_edit.dart

@@ -49,8 +49,6 @@ class _ShipmentRatesEditState extends State<ShipmentRatesEdit> {
 
   @override
   Widget build(BuildContext context) {
-    var shipmentRateModel = Provider.of<ShipmentRateModel>(context);
-
     final minWigBox = InputText(
         labelTextKey: 'rate.min_weight',
         iconData: FontAwesomeIcons.weightHanging,