Why can't I read data from the shared preferences import? - flutter

Question

I have a ListView.builder that builds a certain amount of widgets depending on user input. Each widget has their own specific name and has a DropDownMenu. I save this value with the corresponding name of the widget. It saves it correctly. However, when I try and read the data and create a new list from it, this error appears: [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method '[]' was called on null.
'course' is a list. I am using the shared preferences import. When you tap the flat button, it should build the new list, but it is not. Could you explain to me why this is not working please?
This is code in the main app.
void main() {
runApp(Hemis());
}
class Hemis extends StatefulWidget {
#override
_HemisState createState() => _HemisState();
}
class _HemisState extends State<Hemis> {
_read() async {
final prefs = await SharedPreferences.getInstance();
for(int i = 0; i < course.length; i++) {
listMarks[i].name = course[i].name;
listMarks[i].mark = prefs.getInt(course[i].name) ?? 0;
}
}
#override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
body: SingleChildScrollView(
child: Column(
children: <Widget>[
ListView.builder(
itemCount: course.length,
itemBuilder: (context, index) {
return ModuleListItem(
name: '${course[index].name}',
credits: course[index].credits,
);
},
),
FlatButton(
onPressed: () {
_read();
for(int i = 0; i < course.length; i++) {
print('${listMarks[i].name}: ${listMarks[i].mark}');
}
},
),
],
),
)
)
);
}
}
The widget that is being built.
final percentage = List<String>.generate(100, (i) => "$i");
class ModuleListItem extends StatefulWidget {
const ModuleListItem ({ Key key, this.name, this.credits }): super(key: key);
final String name;
final int credits;
#override
_ModuleListItemState createState() => _ModuleListItemState();
}
class _ModuleListItemState extends State<ModuleListItem> {
String dropdownValue;
bool isSwitched = false;
_save() async {
final prefs = await SharedPreferences.getInstance();
final key = '${widget.name}';
final value = int.parse(dropdownValue);
prefs.setInt(key, value);
print('saved $value');
}
#override
Widget build(BuildContext context) {
return Row(
children: <Widget>[
DropdownButton<String>(
value: dropdownValue,
icon: Icon(Icons.keyboard_arrow_down),
onChanged: (String newValue) {
setState(() {
dropdownValue = newValue;
});
},
items: percentage.map<DropdownMenuItem<String>>((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
),
),
Switch(
value: isSwitched,
onChanged: (value) {
setState(() {
isSwitched = value;
if(isSwitched == true) {
_save();
}
print(isSwitched);
});
},
),
],
),
);
}
}