How to display loading widget until a main widget is loaded - flutter

Question

I am using the animation package to create a popup modal However building the widget inside the model is very noticeably slow and is making making the popup take time to open. I am trying to put a loading indicator when opening the modal then build the widget afterward and just update.
I am lost on how to accomplish that.. it would be highly appreciated if someone could help.
this is the animation package method for the modal
ElevatedButton(
onPressed: () {
showModal<void>(
context: context,
builder: (BuildContext context) {
// building _ExampleAlertDialog takes time
return _ExampleAlertDialog(loading: loading);
},
).then((state) => setState(() => {loading = !loading}));
},
child: const Text('SHOW MODAL'),
),
the _ExampleAlertDialog is supposed to be a listView
class _ExampleAlertDialog extends StatefulWidget {
_ExampleAlertDialog({
this.loading,
});
final bool loading;
#override
__ExampleAlertDialogState createState() => __ExampleAlertDialogState();
}
class __ExampleAlertDialogState extends State<_ExampleAlertDialog> {
#override
Widget build(BuildContext context) {
return Container(
margin: EdgeInsets.symmetric(vertical: 50, horizontal: 20),
child: Expanded(
child: ClipRRect(
borderRadius: BorderRadius.circular(5),
child: Material(
child: Column(
children: [
Container(
decoration: BoxDecoration(
border: Border(
bottom: BorderSide(
color: Theme.of(context).dividerColor))),
padding: EdgeInsets.symmetric(vertical: 10, horizontal: 12),
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: [Icon(Icons.arrow_back), Icon(Icons.refresh)],
),
),
// This is the Listview I am trying to avoid onLoad
Expanded(child: widget.loading == true ? Container():
ListView.builder(
itemCount: 16,
itemBuilder: (BuildContext ctxt, int index) {
return Container(
// width: MediaQuery.of(context).size.width * 1,
child: Row(
children: <Widget>[
Icon(
Icons.radio_button_unchecked,
color: Colors.blue,
size: 12.0,
),
SizedBox(
width: 20.0,
),
Text(
"Travetine",
style: TextStyle(
color: Colors.black,
fontWeight: FontWeight.w400),
),
],
),
);
},
)
)
],
),
),
),
),
);
}
}