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/utils/double_back_to_close_app.dart

@@ -34,7 +34,7 @@ class DoubleBackToCloseApp extends StatefulWidget {
 
 class _DoubleBackToCloseAppState extends State<DoubleBackToCloseApp> {
   /// The last time the user tapped Android's back-button.
-  DateTime _lastTimeBackButtonWasTapped;
+  late DateTime _lastTimeBackButtonWasTapped;
 
   /// Returns whether the current platform is Android.
   bool get _isAndroid => Theme.of(context).platform == TargetPlatform.android;
@@ -59,7 +59,7 @@ class _DoubleBackToCloseAppState extends State<DoubleBackToCloseApp> {
   /// local-history of the current route, in order to handle pop. This is done
   /// by [Drawer], for example, so it can close on pop.
   bool get _willHandlePopInternally =>
-      ModalRoute.of(context).willHandlePopInternally;
+      ModalRoute.of(context)!.willHandlePopInternally;
 
   @override
   Widget build(BuildContext context) {