fix nullability issues (#66418)

packages/flutter/lib/src/cupertino/button.dart

@@ -211,7 +211,7 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
   Widget build(BuildContext context) {
     final bool enabled = widget.enabled;
     final CupertinoThemeData themeData = CupertinoTheme.of(context);
-    final Color? primaryColor = themeData.primaryColor;
+    final Color primaryColor = themeData.primaryColor;
     final Color? backgroundColor = widget.color == null
       ? (widget._filled ? primaryColor : null)
       : CupertinoDynamicColor.resolve(widget.color, context);

packages/flutter/lib/src/widgets/list_wheel_scroll_view.dart

@@ -181,11 +181,7 @@ class ListWheelChildBuilderDelegate extends ListWheelChildDelegate {
   @override
   Widget? build(BuildContext context, int index) {
     if (childCount == null) {
-      final Widget child = builder(context, index);
-      // `child` has a non-nullable return type, but might be null when
-      // running with weak checking, so we need to null check it anyway (and
-      // ignore the warning that the null-handling logic is dead code).
-      // ignore: dead_code
+      final Widget? child = builder(context, index);
       return child == null ? null : IndexedSemantics(child: child, index: index);
     }
     if (index < 0 || index >= childCount!)