fixed error on pdf viewer transition

lib/widget/route_transition_gate.dart

@@ -0,0 +1,59 @@
+import 'package:flutter/material.dart';
+
+import 'app_progress_indicator.dart';
+
+/// Delays building [builder] until the enclosing route's enter animation has
+/// finished. Some widgets (notably `SfPdfViewer`) call `localToGlobal` during
+/// their first layout and assert with `RenderBox was not laid out` when an
+/// ancestor page-transition `RenderTransform` still has no size mid-push.
+/// Gating the mount behind the settled animation avoids that race.
+class RouteTransitionGate extends StatefulWidget {
+  const RouteTransitionGate({super.key, required this.builder, this.placeholder});
+
+  final WidgetBuilder builder;
+
+  /// Shown while the route is still animating in. Defaults to a centered
+  /// large progress indicator.
+  final Widget? placeholder;
+
+  @override
+  State<RouteTransitionGate> createState() => _RouteTransitionGateState();
+}
+
+class _RouteTransitionGateState extends State<RouteTransitionGate> {
+  bool _ready = false;
+  Animation<double>? _routeAnimation;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    if (_ready || _routeAnimation != null) return;
+    final animation = ModalRoute.of(context)?.animation;
+    if (animation == null || animation.isCompleted) {
+      _ready = true;
+      return;
+    }
+    _routeAnimation = animation..addStatusListener(_onAnimationStatus);
+  }
+
+  void _onAnimationStatus(AnimationStatus status) {
+    if (status == AnimationStatus.completed && mounted) {
+      setState(() => _ready = true);
+    }
+  }
+
+  @override
+  void dispose() {
+    _routeAnimation?.removeStatusListener(_onAnimationStatus);
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (!_ready) {
+      return widget.placeholder ??
+          const Center(child: AppProgressIndicator.large());
+    }
+    return widget.builder(context);
+  }
+}