Null check operator used on a null value in a PageView on a PageController in Flutter

Question

So I have a PageController appended to a PageView, I set the value of the PageController while declaring it globally but I came across an error, Null check operator used on a null value, so I called the initState method and initialised the value of my controller there but nothing changed, I have looked at another question on stack overflow, with the same error but I don't think it is related to my problem. Anyway, here is the code and the error:

Error:

The following _CastError was thrown building:
Null check operator used on a null value

When the exception was thrown, this was the stack: 
#0      ScrollPosition.minScrollExtent (package:flutter/src/widgets/scroll_position.dart:136:49)
#1      _PagePosition.page (package:flutter/src/widgets/page_view.dart:386:22)
#2      PageController.page (package:flutter/src/widgets/page_view.dart:179:21)
#3      _HomeState.build.<anonymous closure>.<anonymous closure> (package:.../screens/home.dart:160:43)
#4      SliverChildBuilderDelegate.build (package:flutter/src/widgets/sliver.dart:455:22)
...

Code:

PageController _pageController = PageController(initialPage: 1, viewportFraction: 0.8);

  @override
  void initState() {
    super.initState();
    _pageController = PageController(initialPage: 1, viewportFraction: 0.8);
  }

  @override
  Widget build(BuildContext context) {
   // other code...
   return PageView.builder(
     controller: _pageController,
     itemCount: (list.length + 1),
     scrollDirection: Axis.vertical,
     onPageChanged: (val) {
       setState(() {});
     },
     itemBuilder: (context, index) {
       if (index == list.length) {
                    // Widget
        }
       else {
           // other code
           _pageController.page.toInt() != (index) ? (Widget) : (Widget) // line where the error occurs
        }
      },
    );
  }

Sometimes the error resolves by itself, and returns the PageView perfectly, but that's rare, happens only a few times.

EDIT: I forgot to mention but, once I start scrolling, the problem resolves