Make CupertinoNavigationBarBackButton correctly return an assert error (#30815)

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

@@ -1231,7 +1231,7 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
   Widget build(BuildContext context) {
     final ModalRoute<dynamic> currentRoute = ModalRoute.of(context);
     assert(
-      currentRoute.canPop,
+      currentRoute?.canPop == true,
       'CupertinoNavigationBarBackButton should only be used in routes that can be popped',
     );
 

packages/flutter/test/cupertino/nav_bar_test.dart

@@ -883,6 +883,28 @@ void main() {
     );
     expect(SystemChrome.latestStyle, SystemUiOverlayStyle.dark);
   });
+
+  testWidgets('CupertinoNavigationBarBackButton shows an error when manually added outside a route', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const CupertinoNavigationBarBackButton()
+    );
+
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
+    expect(exception.toString(), contains('CupertinoNavigationBarBackButton should only be used in routes that can be popped'));
+  });
+
+  testWidgets('CupertinoNavigationBarBackButton shows an error when placed in a route that cannot be popped', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const CupertinoApp(
+          home: CupertinoNavigationBarBackButton(),
+        ),
+      );
+
+      final dynamic exception = tester.takeException();
+      expect(exception, isAssertionError);
+      expect(exception.toString(), contains('CupertinoNavigationBarBackButton should only be used in routes that can be popped'));
+  });
 }
 
 class _ExpectStyles extends StatelessWidget {