The following assertion was thrown while handling a gesture:
PageController.page cannot be accessed before a PageView is built with
it. 'package:flutter/src/widgets/page_view.dart': Failed assertion: line
96 pos 7:'positions.isNotEmpty'

...
PageController _pageController;

@override
void initState{
    _pageController = PageController(initialPage: 100, viewportFraction: 0.2);
}

@override
Widget build(BuildContext context) {
  return Center(
    child: FlatButton(
        onPressed: (){
         print(_pageController.page);
      }
    )
  );
}