flutter scheduler callback errors? - flutter

Question

So I'm running through some tutorials for Flutter Animation, and I had to use something called a Post Frame callback to ensure that function didn't run until after the build function, like so (with the function in question displayed):
void initState() {
super.initState();
WidgetsBinding.instance.addPostFrameCallback((_) {
_addTrips();
});
}
void _addTrips() {
// get data from db
List<Trip> _trips = [
Trip(title: 'Beach Paradise', price: '350', nights: '3', img: 'beach.png'),
Trip(title: 'City Break', price: '400', nights: '5', img: 'city.png'),
Trip(title: 'Ski Adventure', price: '750', nights: '2', img: 'ski.png'),
Trip(title: 'Space Blast', price: '600', nights: '4', img: 'space.png'),
];
The build function (within the same initState) looks like this:
#override
Widget build(BuildContext context) {
return AnimatedList(
key: _listKey,
initialItemCount: _tripTiles.length,
itemBuilder: (context, index, animation) {
return SlideTransition(
child: _tripTiles[index],
position: animation.drive(_offset)
);
}
);
}
The items on the list fail to populate when I run the code. The error that I'm getting in the run panel is the following with the stack included:
Exception caught by scheduler library
The following assertion was thrown during a scheduler callback:
'package:flutter/src/widgets/animated_list.dart': Failed assertion: line 963 pos 12:
'itemIndex >= 0 && itemIndex <= _itemsCount': is not true.
Either the assertion indicates an error in the framework itself, or we should provide
substantially more information in this error message to help you determine and fix the
underlying
cause.
In either case, please report this assertion by filing a bug on GitHub:
https://github.com/flutter/flutter/issues/new?template=2_bug.md
When the exception was thrown, this was the stack:
#2 SliverAnimatedListState.insertItem
(package:flutter/src/widgets/animated_list.dart:963:12)
#3 AnimatedListState.insertItem
(package:flutter/src/widgets/animated_list.dart:484:42)
#4 _TripListState._addTrips.<anonymous closure>
(package:ninja_trips/shared/tripList.dart:33:29)
#5 List.forEach (dart:core-patch/growable_array.dart:403:8)
#6 _TripListState._addTrips (package:ninja_trips/shared/tripList.dart:31:12)
#7 _TripListState.initState.<anonymous closure>
(package:ninja_trips/shared/tripList.dart:18:7)
#8 SchedulerBinding._invokeFrameCallback
(package:flutter/src/scheduler/binding.dart:1143:15)
#9 SchedulerBinding.handleDrawFrame
(package:flutter/src/scheduler/binding.dart:1088:9)
#10 SchedulerBinding.scheduleWarmUpFrame.<anonymous closure>
(package:flutter/src/scheduler/binding.dart:863:7)
(elided 13 frames from class _AssertionError, class _RawReceivePortImpl, class _Timer,
dart:async, and dart:async-patch)
The tutorial in question didn't seem to anticipate this as a possibility and I'm pretty stumped so far. Any ideas?
Thanks,
ATD