Fix errors in RadialExpansion class example (#12762)

This change fixes these errors  
<img width="722" height="78" alt="err_1"
src="https://github.com/user-attachments/assets/cd1aef4b-2d3f-4987-a23f-b7592b4d2ebf"
/>

<img width="623" height="40" alt="err_2"
src="https://github.com/user-attachments/assets/447e89e6-e5cf-4847-ba82-56a3bdf6e742"
/>

Fixes #12761

Author: AbdeMohlbi

src/content/ui/animations/hero-animations.md

@@ -561,8 +561,8 @@ class RadialExpansion extends StatelessWidget {
   }) : [!clipRectSize = 2.0 * (maxRadius / math.sqrt2);!]
 
   final double maxRadius;
-  final clipRectSize;
-  final Widget child;
+  final double clipRectSize;
+  final Widget? child;
 
   @override
   Widget build(BuildContext context) {