LateInitializationError: Field 'controller' has not been initialized - flutter

Question

Im using a SideBar() widget inside the SideBarWidget() and using it in my main screen SideBarScreen()
but i cant initialize the controller in the Sidebar() widget... How can i fix this
sidebar_Screen
class _SideBarScreenState extends State<SideBarScreen> {
#override
Widget build(BuildContext context) {
return SafeArea(
child: Stack(
children: [
ScaffoldScreen(
iconButton: IconButton(
onPressed: () {SideBarWidget().toggle();},
icon: kScaffoldScreenButtonIcon,
),
),
SideBarWidget(),
],
),
);
}
}
sideBarWidget
class SideBarWidget extends StatelessWidget {
SideBarWidget({Key? key}) : super(key: key);
void toggle() {
SideBarState().toggle();
}
late final SideBar sideBarWidget = SideBar();
#override
Widget build(BuildContext context) {
return sideBarWidget;
}
}
SideBar
class SideBar extends StatefulWidget {}
class SideBarState extends State<SideBar> with SingleTickerProviderStateMixin{
late AnimationController controller;
late Animation<Offset> _offsetAnimation;
#override
void initState() {
super.initState();
controller = AnimationController(vsync: this, duration: const Duration(seconds: 2));
_offsetAnimation = Tween<Offset>(
begin: const Offset(-1.0, 0.0),
end: const Offset(0.0, 0.0),
).animate(CurvedAnimation(parent: controller, curve: Curves.easeOut));
}
#override
void dispose() {
controller.dispose();
super.dispose();
}
void toggle() {
if (controller.isCompleted) {
controller.reverse();
}
else {controller.forward();}
}
#override
Widget build(BuildContext context) {
return SlideTransition();
}
}
This gives the error LateInitializationError: Field 'controller' has not been initialized.
I tried passing the controller as a parameter to the SideBar() but it gives off an exception:
This widget has been unmounted, so the State no longer has a context (and should be considered defunct).

Answer 1

The problem is that you're creating a new instance of SideBar every time you call SideBarWidget().toggle(). Instead, you should be using the same instance of SideBar so that the state is retained.
class SideBarWidget extends StatelessWidget {
SideBarWidget({Key? key}) : super(key: key);
final sideBar = SideBar();
void toggle() {
sideBar.state.toggle();
}
#override
Widget build(BuildContext context) {
return sideBar;
}
}
So now you are using the same instance of SideBar every time and you don't need to pass any parameters.
Note:
To use like this, you also need to change the SideBar class to a StatefulWidget
class SideBar extends StatefulWidget {
#override
_SideBarState createState() => _SideBarState();
}