Makes AutomaticKeepAlive.child non null (#101376)

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

@@ -28,13 +28,13 @@ class AutomaticKeepAlive extends StatefulWidget {
   /// [KeepAlive] widget appropriately.
   const AutomaticKeepAlive({
     Key? key,
-    this.child,
+    required this.child,
   }) : super(key: key);
 
   /// The widget below this widget in the tree.
   ///
   /// {@macro flutter.widgets.ProxyWidget.child}
-  final Widget? child;
+  final Widget child;
 
   @override
   State<AutomaticKeepAlive> createState() => _AutomaticKeepAliveState();
@@ -42,7 +42,7 @@ class AutomaticKeepAlive extends StatefulWidget {
 
 class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
   Map<Listenable, VoidCallback>? _handles;
-  Widget? _child;
+  late Widget _child;
   bool _keepingAlive = false;
 
   @override
@@ -60,7 +60,7 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
   void _updateChild() {
     _child = NotificationListener<KeepAliveNotification>(
       onNotification: _addClient,
-      child: widget.child!,
+      child: widget.child,
     );
   }
 
@@ -228,10 +228,9 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
 
   @override
   Widget build(BuildContext context) {
-    assert(_child != null);
     return KeepAlive(
       keepAlive: _keepingAlive,
-      child: _child!,
+      child: _child,
     );
   }