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/parabolic_animation_widget.dart

@@ -11,7 +11,7 @@ class ParabolicAnimationWidget extends AnimatedWidget {
   final Offset startAdjustOffset;
   final Offset endAdjustOffset;
 
-  ParabolicAnimationWidget({
+  late ParabolicAnimationWidget({
     @required Animation<double> animation,
     @required this.stackKey,
     @required this.startKey,
@@ -66,17 +66,17 @@ class ParabolicAnimationWidget extends AnimatedWidget {
 
   void _calPoints() {
     if (_startOffset == null) {
-      RenderBox stackBox = stackKey.currentContext.findRenderObject();
+      RenderBox stackBox = stackKey.currentContext!.findRenderObject();
       Offset stackBoxOffset = stackBox.globalToLocal(Offset.zero);
 
       EdgeInsets startMargin = _margin(startKey);
-      RenderBox startBox = startKey.currentContext.findRenderObject();
+      RenderBox startBox = startKey.currentContext!.findRenderObject();
       _startOffset = startBox.localToGlobal(Offset(
           startMargin.left + startAdjustOffset.dx,
           stackBoxOffset.dy + startMargin.top + startAdjustOffset.dy));
 
       EdgeInsets endMargin = _margin(endKey);
-      RenderBox endBox = endKey.currentContext.findRenderObject();
+      RenderBox endBox = endKey.currentContext!.findRenderObject();
       _endOffset = endBox.localToGlobal(Offset(
           endMargin.left + endAdjustOffset.dx,
           stackBoxOffset.dy + endMargin.top + endAdjustOffset.dy));
@@ -84,7 +84,7 @@ class ParabolicAnimationWidget extends AnimatedWidget {
   }
 
   EdgeInsets _margin(GlobalKey key) {
-    final Widget widget = key.currentContext.widget;
+    final Widget widget = key.currentContext!.widget;
     EdgeInsets margin = (widget is Container) ? widget.margin : EdgeInsets.zero;
     return margin ?? EdgeInsets.zero;
   }