[scrollable_positioned_list] Failed assertion: line 239 pos 12: '_scrollableListState == null': is not true in an Overlay

[edwardaux]

Problem description

Using ^0.2.0-nullsafety.0 of scrollable_positioned_list inside a stateful widget that displays an overlay, I constantly get this error every time the overlay gets re-created:

════════ Exception caught by widgets library ═══════════════════════════════════
The following assertion was thrown building PageStorage:
'package:scrollable_positioned_list/src/scrollable_positioned_list.dart': Failed assertion: line 239 pos 12: '_scrollableListState == null': is not true.

Steps to reproduce

My particular use case is a stateful widget that is offering typeahead suggestions (somewhat like what Spotlight does on macOS) using an Overlay.

The underlying problem is a combo of a few things:

• I'm using a stateful widget with an _itemScrollController instance variable
• I'm assigning a new ItemScrollController instance to _itemScrollController in my widget's initState() function
• Every time the user types a key, the Overlay gets recreated
• That _itemScrollController object is passed into the ScrollablePositionedList when the overlay gets rebuilt

Under the covers, though, the scroll controller doesn't like it if it is being re-attached to a new widget when it had previously been attached to another one.

Expected behavior

Well, I wasn't expecting a crash... however, if you think about the above scenario, I'm not entirely sure how I'd expect this library to be able to cope with re-attaching.

Not sure if there's an underlying bug here where this package could handle this scenario a bit better, but I'm raising this ticket as a way of sharing what the underlying problem is and how to resolve (in case other developers stumble across the same problem).

Steps to resolve

While technically 100% accurate, the stack trace isn't particularly helpful in terms of figuring out /why/ it is in this state. My "fix" was relatively simple... just before re-creating the Overlay, I just create a new scroll controller. eg:

_itemScrollController = ItemScrollController();
... // create overlay

[felipecastrosales]

I'm have a same problem here. 🤔

[felipecastrosales]

But my error show inside tests.

My mixin:

mixin ScrollToMixin {
  void scrollTo(int index, ItemScrollController itemScrollController) {
    itemScrollController.scrollTo(
      index: index,
      alignment: 0,
      curve: Curves.easeIn,
      duration: const Duration(milliseconds: 1500),
    );
  }
}

[CodeKiteshub]

getting same error.

[DevTiago]

+1

[andy-thestunt]

I am also facing the same issue.