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/general/animation_point_manager.dart

@@ -5,8 +5,8 @@ import 'popup_animation_widget.dart';
 
 class AnimationPointManager {
   List<AnimatedWidget> list = [];
-  static AnimationController controller1;
-  static AnimationController controller2;
+  static late AnimationController controller1;
+  static late AnimationController controller2;
 
   Future<void> addParabolicAniamtion({
     @required TickerProvider vsync,
@@ -55,9 +55,9 @@ class AnimationPointManager {
     @required GlobalKey stackKey,
     @required GlobalKey startKey,
     @required Widget child,
-    Duration duration,
+    Duration? duration,
     Offset popupOffset = Offset.zero,
-    AnimationStatusListener statusListener,
+    AnimationStatusListener? statusListener,
   }) async {
     controller2 = createController(vsync, duration);