Widget build(BuildContext context) { if (_controller.hasClients) { SchedulerBinding.instance.addPostFrameCallback((_) => _controller.jumpTo( 0.0 )); return Container( child : ...

エラーログ

そのままやると:

  Widget build(BuildContext context) {
    if (_controller.hasClients) {
      _controller.jumpTo(0.0);
Performing hot reload...
Syncing files to device iPhone XR...
Reloaded 1 of 553 libraries in 949ms.
flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following assertion was thrown building MyWidget(dirty, dependencies:
flutter: [_LocalizationsScope-[GlobalKey#ab210], _InheritedTheme], state: _MyWidgetState#100b7):
flutter: Unexpected call to replaceSemanticsActions() method of RawGestureDetectorState.
flutter: The replaceSemanticsActions() method can only be called outside of the build phase.