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/popup_animation_widget.dart

@@ -9,7 +9,7 @@ class PopupAnimationWidget extends AnimatedWidget {
   final Offset popupOffset;
   final Animation<double> animation;
 
-  PopupAnimationWidget({
+  late PopupAnimationWidget({
     @required this.animation,
     @required this.stackKey,
     @required this.startKey,
@@ -49,11 +49,11 @@ class PopupAnimationWidget extends AnimatedWidget {
 
   void _calAnimation() {
     if (_startOffset == null) {
-      final RenderBox stackBox = stackKey.currentContext.findRenderObject();
+      final RenderBox stackBox = stackKey.currentContext!.findRenderObject();
       final Offset stackBoxOffset = stackBox.globalToLocal(Offset.zero);
 
       final EdgeInsets startMargin = _margin(startKey);
-      final RenderBox startBox = startKey.currentContext.findRenderObject();
+      final RenderBox startBox = startKey.currentContext!.findRenderObject();
 
       _startOffset = startBox.localToGlobal(Offset(
           startMargin.left + popupOffset.dx,
@@ -62,7 +62,7 @@ class PopupAnimationWidget extends AnimatedWidget {
   }
 
   EdgeInsets _margin(GlobalKey key) {
-    final Widget widget = key.currentContext.widget;
+    final Widget widget = key.currentContext!.widget;
     final EdgeInsets margin =
     (widget is Container) ? widget.margin : EdgeInsets.zero;
     return margin ?? EdgeInsets.zero;