Fix null check for content dimensions in page getter (#87824)

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

@@ -385,10 +385,12 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
   @override
   double? get page {
     assert(
-      !hasPixels || (minScrollExtent != null && maxScrollExtent != null),
+      !hasPixels || hasContentDimensions,
       'Page value is only available after content dimensions are established.',
     );
-    return !hasPixels ? null : getPageFromPixels(pixels.clamp(minScrollExtent, maxScrollExtent), viewportDimension);
+    return !hasPixels || !hasContentDimensions
+      ? null
+      : getPageFromPixels(pixels.clamp(minScrollExtent, maxScrollExtent), viewportDimension);
   }
 
   @override

packages/flutter/test/widgets/page_view_test.dart

@@ -14,6 +14,12 @@ import 'states.dart';
 const Duration _frameDuration = Duration(milliseconds: 100);
 
 void main() {
+  testWidgets('PageController cannot return page while unattached',
+      (WidgetTester tester) async {
+    final PageController controller = PageController();
+    expect(() => controller.page, throwsAssertionError);
+  });
+
   testWidgets('PageView control test', (WidgetTester tester) async {
     final List<String> log = <String>[];