Trying to use showDialog()/show Pop up on app startup - flutter

Question

What I want to achieve: I want to open a pop up explaining my app when it starts.
My approach: As far as I understand it from googling the issue, I should use the showDialog() method. In its most basic form:
showDialog(
context: context,
builder: (context) {
return Text('data');
});
I tried returning actual dialogs (e.g. AlertDialog) but it doesn't change the behavior so I'm just using Text() with a string as a placeholder for now.
The problem:
No matter where I place the showDialog function, it doesn't work as intended (also see scrennshots below):
Placing it in initState: I get an error message about inherited Widgets being called before the initState is done + an explanation about dependiencies I can barely follow.
Placing it in the build method: I get an error message that setState() or markNeedsBuild() gets called while the app is already buildung widgets.
Placing it in DidChangeAppLifeCycleState(): This is actually working and opening the pop when I pause the app and then resume it. It is not opening on app startup though.
Wrapping it in WidgetsBinding.instance!.addPostFrameCallback(): An idea I picked up here: How to show a popup on app start in Flutter. Doesn't change the outcome of the error messages, neither in initState nor in build.
Any ideas?
Screenshots:
From initState:
From build method:
From DidChangeAppLifecycleState (the "succesful" variant:

Answer 1

Will you please try below code in your init method? I hope this may work.
Future.delayed(Duration.zero, () async {
myFunction();
});

Answer 2

Using WidgetsBinding.instance.addPostFrameCallback inside initState perform its inner task after the 1st frame is complete.
addPostFrameCallback Schedule a callback for the end of this frame.
Next issue arise for not having material. You can directly return AlertDialog on builder or wrap with any material widget like Material, Scaffold..
#override
void initState() {
super.initState();
WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
showDialog(
context: context,
builder: (context) {
return const AlertDialog(
content: Text('data'),
);
},
);
});
}
If you are running version<3 null safety, use WidgetsBinding.instance?.addPostFrameCallback

Answer 3

One of the methods with WidgetsBinding.instance!.addPostFrameCallback() works fine .
If you show a normal show dialog with the press of a button too it will produce the same result.
Here, you need to wrap the text("data") in a dialog widget such as alertDialog or simpleDialog widget as needed and it will display the dialog within the current scaffold as -
WidgetsBinding.instance!.addPostFrameCallback((_) async {
return await showDialog(
context: context,
builder: (context) {
return AlertDialog(
content: Text("data"),
);
});
});
I tried adding this in the init state and the dialog pops up fine when I restart the app

Answer 4

Thanks a lot for your answers. I ficed the issue by rewriting with your suggestions; and it works. I tihnk the issue was that I did not have _ or anything else in my WidgetsBinding code. So I did:
WidgetsBinding.instance?.addPostFrameCallback(() {})
instead of
WidgetsBinding.instance?.addPostFrameCallback((_) {})