Flutter Mobx - setState() or markNeedsBuild() called during build

When I try to navigate from a page to another, I'm getting the error bellow. After some research i found some solutions as calling the navigation inside a SchedulerBinding.instance.addPostFrameCallback, but even if that solve in some cases, for this specific case none of them are working.

I'm using an IndexedStack with three AnimatedOpacity wrapping one different page. Besides the error, could be the way on that the IndexedStack renders its content? Seems to be cause it renders all the pages at the same time while shows only the current index. But how i can solve this if a i need switch between these pages with the same state? I tried with PageView but the state is miss when change from one page to another.

Anyone can help me on this?

════════ Exception caught by flutter_mobx ══════════════════════════════════════
The following MobXCaughtException was thrown:
setState() or markNeedsBuild() called during build.
This Observer widget cannot be marked as needing to build because the framework is already in the process of building widgets.  A widget can be marked as needing to be built during the build phase only if one of its ancestors is currently building. This exception is allowed because the framework builds parent widgets before children, which means a dirty descendant will always be built. Otherwise, the framework might not visit this widget during this build phase.
The widget on which setState() or markNeedsBuild() was called was:
  Observer
The widget which was currently being built when the offending call was made was:
  AnimatedOpacity

When the exception was thrown, this was the stack
#0      Element.markNeedsBuild.<anonymous closure> 
package:flutter/…/widgets/framework.dart:4167
#1      Element.markNeedsBuild 
package:flutter/…/widgets/framework.dart:4182
#2      ObserverElementMixin.invalidate 
package:flutter_mobx/src/observer_widget_mixin.dart:70
#3      ReactionImpl._run 
package:mobx/…/core/reaction.dart:119
#4      ReactiveContext._runReactionsInternal 
package:mobx/…/core/context.dart:345
...

The build method:

@override
  Widget build(BuildContext context) {
    return IndexedStack(
      index: index,
      children: <Widget>[
        AnimatedOpacity(
          key: Key('animatedOpacity_0'),
          opacity: index == 0 ? 1.0 : 0.0,
          duration: Duration(milliseconds: 500),
          child: Page1(),
        ),
        AnimatedOpacity(
          key: Key('animatedOpacity_1'),
          opacity: index == 1 ? 1.0 : 0.0,
          duration: Duration(milliseconds: 500),
          child: Page2(),
        ),
        AnimatedOpacity(
          key: Key('animatedOpacity_2'),
          opacity: index == 2 ? 1.0 : 0.0,
          duration: Duration(milliseconds: 500),
          child: Page3(),
        ),
      ],
    );
  }

UPDATE:

The problem occurs on the third AnimatedOpacity, and if i remove the AnimatedOpacity widgets, the problems doesn't occurs but i can't figure it out why. Someone has some explanation about that?