content vanish every time I restart the app - flutter

Question

I created a page where I can add exercise for my classes and in this page students can create their custom training (it's a gym app), the only problem is that every time the user closes the app all the training is gone.
I leave below my code, I've done some researches and I kind of understood that there is something with SharPreferences but all the tutorial talk about a login page which is not my case.
can you please advise?? I leave my code below, thanks!
import 'package:flutter/material.dart';
import 'package:flutter_slidable/flutter_slidable.dart';
class Custom extends StatefulWidget {
#override
_CustomState createState() => new _CustomState();
}
class _CustomState extends State<Custom> {
int _count = 0;
#override
Widget build(BuildContext context) {
List<Widget> _esercizi = List.generate(_count, (int i) => Esercizi());
bool visible = _count > 0;
return GestureDetector(
onTap: () {
FocusScope.of(context).requestFocus(FocusNode());
},
child: Scaffold(
appBar: AppBar(
title: Text('Scheda personalizzata'),
actions: [
Visibility(
visible: visible,
child: IconButton(
icon: Icon(Icons.remove_rounded),
onPressed: _removeNewEsercizi,
),
),
IconButton(icon: Icon(Icons.add), onPressed: _addNewEsercizi),
],
backgroundColor: Colors.blueGrey,
),
body: LayoutBuilder(builder: (context, constraint) {
return Padding(
padding: const EdgeInsets.all(10.0),
child: SingleChildScrollView(
child: Column(
mainAxisAlignment: MainAxisAlignment.start,
children: <Widget>[
Container(
height: 700,
child: ListView(
scrollDirection: Axis.vertical,
children: _esercizi,
),
),
//new ContactRow()
],
),
),
);
}),
),
);
}
void _addNewEsercizi() {
setState(() {
_count = _count + 1;
});
}
void _removeNewEsercizi() {
setState(() {
_count = _count - 1;
});
}
}
class Esercizi extends StatefulWidget {
#override
State<StatefulWidget> createState() => _Esercizi();
}
class _Esercizi extends State<Esercizi> {
String dropdownValue = 'Seleziona esercizio';
#override
Widget build(BuildContext context) {
Size size = MediaQuery.of(context).size;
return Padding(
padding: const EdgeInsets.all(2.0),
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
DropdownButton<String>(
value: dropdownValue,
style: TextStyle(color: Colors.blueGrey),
onChanged: (String newValue) {
setState(() {
dropdownValue = newValue;
});
},
items: <String>['Seleziona esercizio', 'One', 'Two', 'Three', 'Fo'
'ur', 'Five', 'Six']
.map<DropdownMenuItem<String>>((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
),
Container(
height: 50,
width: 90,
child: Center(
child: Padding(
padding: const EdgeInsets.all(8.0),
child: TextField(
cursorColor: Colors.white,
decoration: InputDecoration(
hintText: 'Time/Reps',
hintStyle: TextStyle(color: Colors.blueGrey),
),
keyboardType: TextInputType.number,
),
),
),
),
],
),
);
}
}