Flutter - Cannot modify List<Widget> - list

Question

I have a fixed-length List that is used in the "build" function. I want to replace one element in that List with another. Both elements in the List are Containers. I replace the element in the List and do a setState(). The build function uses the List in the body of the Scaffold. I get no error, however when the rebuild is done, nothing has changed.
To my knowledge this is not breaking any rules and as far as I know should cause no problems.
While I could code around the issue, I think that with something as fundamental as this, I need to find the reason for it.
Any ideas as to why this is happening?
Code added below:
Below is the code that DOES NOT work
if (_lwDisplay[iNdxDisplay] == null) /*INITIAL DISPLAY */ {
_lwDisplay[iNdxDisplay] = wContainer;
} else {
setState(() {
_tfDataHasChanged = true;
_lwDisplay[iNdxDisplay] = wContainer;
});
}
Below is the code that DOES work
if (_lwDisplay[iNdxDisplay] == null) /*INITIAL DISPLAY */ {
_lwDisplay[iNdxDisplay] = wContainer;
} else {
_tfDataHasChanged = _fnHasDataChanged() /* FOR FAB */;
List<Widget> lwDisplay2 = List(_lwDisplay.length);
for (int iNdx = 0; iNdx < lwDisplay2.length; iNdx++) {
if (iNdx != iNdxDisplay) {
lwDisplay2[iNdx] = _lwDisplay[iNdx];
}
}
lwDisplay2[iNdxDisplay] = wContainer;
_lwDisplay = lwDisplay2;
setState(() {});
}

Answer 1

Without seeing the entirety of code, and according to this SO question,
Move the instantiation of the list of widgets to the build method of your main widget.
I.E.
class MainWidgetState extends State<MainWidget> {
List<YOUR_WIDGET> _lwDisplay = new List();
Widget build(BuildContext context) {
//Create your list
}
}
Also, add a key to each item in your widget (if you haven't done so already).