docs: Document WidgetSize widget

lib/ui/helpers/widget_size.dart

@@ -1,14 +1,26 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart';
 
+/// A helper widget that calls a callback when its size changes.
+///
+/// This is useful when you want to know the size of a widget, and use it in
+/// another leaf of the tree.
+///
+/// The [onChange] callback is called after the widget is rendered, and the
+/// size of the widget is different from the previous render.
 class WidgetSize extends StatefulWidget {
+  /// Creates a helper widget that calls a callback when its size changes.
   const WidgetSize({
     required this.onChange,
     required this.child,
     super.key,
   });
+
+  /// The child widget, the size of which is to be measured.
   final Widget child;
-  final Function onChange;
+
+  /// The callback to be called when the size of the widget changes.
+  final Function(Size) onChange;
 
   @override
   State<WidgetSize> createState() => _WidgetSizeState();
@@ -34,6 +46,11 @@ class _WidgetSizeState extends State<WidgetSize> {
     }
 
     final newSize = context.size;
+
+    if (newSize == null) {
+      return;
+    }
+
     if (oldSize == newSize) {
       return;
     }