phase3: nullfix.py batch script — bang/late/param-nullable

Automated null-safety fixes driven by dart analyze (no corruption):
- unchecked_use_of_nullable_value: insert '!' on receiver (property/method/[]/op)
- not_initialized field/var: mark 'late'
- missing_default_value_for_parameter: nullable param
Errors: 2374(peak) -> 907

lib/widgets/mobile/mobile_product_item.dart

@@ -98,7 +98,7 @@ class MobileProductItemState extends State<MobileProductItem> {
                           new Container(
                             child: widget.business.showMonthlySold ?
                             Text(
-                              S.of(context).sold_per_month_token(widget.product.monthSales.toStringAsFixed(0)),
+                              S.of(context).sold_per_month_token(widget.product.monthSales!.toStringAsFixed(0)),
                               style: new TextStyle(
                                   fontSize: 9.0
                               ),