Changing ElevatedButton.child to be non-nullable (#87901)

packages/flutter/lib/src/material/elevated_button.dart

@@ -96,7 +96,7 @@ class ElevatedButton extends ButtonStyleButton {
     FocusNode? focusNode,
     bool autofocus = false,
     Clip clipBehavior = Clip.none,
-    required Widget? child,
+    required Widget child,
   }) : super(
     key: key,
     onPressed: onPressed,

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

@@ -7157,7 +7157,7 @@ class IgnorePointer extends SingleChildRenderObjectWidget {
 ///         height: 100.0,
 ///         child: ElevatedButton(
 ///           onPressed: () {},
-///           child: null,
+///           child: const Text('Lower'),
 ///         ),
 ///       ),
 ///       SizedBox(
@@ -7170,7 +7170,7 @@ class IgnorePointer extends SingleChildRenderObjectWidget {
 ///               primary: Colors.blue.shade200,
 ///             ),
 ///             onPressed: () {},
-///             child: null,
+///             child: const Text('Upper'),
 ///           ),
 ///         ),
 ///       ),

packages/flutter/test/material/material_test.dart

@@ -244,7 +244,7 @@ void main() {
                 onPressed: () {
                   pressed = true;
                 },
-                child: null,
+                child: const Text('Button'),
               ),
               const Material(
                 type: MaterialType.transparency,

packages/flutter/test/widgets/widget_inspector_test.dart

@@ -287,7 +287,7 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
       final GlobalKey topButtonKey = GlobalKey();
 
       Widget selectButtonBuilder(BuildContext context, VoidCallback onPressed) {
-        return Material(child: ElevatedButton(onPressed: onPressed, key: selectButtonKey, child: null));
+        return Material(child: ElevatedButton(onPressed: onPressed, key: selectButtonKey, child: const Text('Button')));
       }
       // State type is private, hence using dynamic.
       dynamic getInspectorState() => inspectorKey.currentState;
@@ -390,7 +390,7 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
       final GlobalKey inspectorKey = GlobalKey();
 
       Widget selectButtonBuilder(BuildContext context, VoidCallback onPressed) {
-        return Material(child: ElevatedButton(onPressed: onPressed, key: selectButtonKey, child: null));
+        return Material(child: ElevatedButton(onPressed: onPressed, key: selectButtonKey, child: const Text('Button')));
       }
       // State type is private, hence using dynamic.
       dynamic getInspectorState() => inspectorKey.currentState;
@@ -590,7 +590,7 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
 
       InspectorSelectButtonBuilder selectButtonBuilder(Key key) {
         return (BuildContext context, VoidCallback onPressed) {
-          return Material(child: ElevatedButton(onPressed: onPressed, key: key, child: null));
+          return Material(child: ElevatedButton(onPressed: onPressed, key: key, child: const Text('Button')));
         };
       }