Flutter: The Method was called on null - flutter

Question

I want to run my App,but im getting this error after I tried to reload
it. But i keep getting this error. I tried to remove some code that i
edited befor it stopped working but i keep getting this error. This Code is from the following link and i changed it a bit. "https://pub.dev/packages/flutter_circular_slider" Cansomeone help me?
error Message
My Error Message in the Terminal:
The method '>' was called on null. Receiver: null Tried calling: >(null)
class Register extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
home: RegisterLayout(),
);
}
}
class RegisterLayout extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
body: Container(
child: RegisterPage(),
),
);
}
}
class RegisterPage extends StatefulWidget {
#override
_RegisterThirdPageState createState() => _RegisterThirdPageState();
}
class _RegisterThirdPageState extends State<RegisterPage> {
int initTime;
int endTime;
int inBedTime;
int outBedTime;
String startReading;
String endReading;
String overTime;
#override
void _updateLabels(int init, int end, int all) {
setState(() {
inBedTime = init;
outBedTime = end;
});
}
#override
Widget build(BuildContext context) {
return Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
SizedBox(height: 20),
Text(
'How long did you want to read',
style: TextStyle(color: Colors.green[600], fontSize: 22),
),
DoubleCircularSlider(
288,
initTime,
endTime,
height: 300.0,
width: 300.0,
primarySectors: 4,
baseColor: Colors.grey[200],
selectionColor: Colors.green[400],
handlerColor: Colors.green[600],
handlerOutterRadius: 20.0,
onSelectionChange: _updateLabels,
sliderStrokeWidth: 40.0,
child: Padding(
padding: const EdgeInsets.all(42.0),
child: Center(
child: Text('${_formatIntervalTime(inBedTime, outBedTime)}',
style: TextStyle(fontSize: 40.0, color: Colors.green[600]))),
),
),
Row(mainAxisAlignment: MainAxisAlignment.spaceEvenly, children: [
_formatBedTime('START', inBedTime),
_formatBedTime('END', outBedTime),
]),
FlatButton(
padding: EdgeInsets.only(top: 10, bottom: 10, left: 100, right: 100),
child: Text('N E X T', style: TextStyle(fontSize: 30),),
color: Colors.green[400],
textColor: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(50.0),
),
onPressed: () {
startReading = _formatTime(inBedTime);
endReading = _formatTime(outBedTime);
overTime = _formatIntervalTime(inBedTime, outBedTime);
print(startReading);
print(endReading);
print(overTime);
},
),
],
);
}
Widget _formatBedTime(String pre, int time) {
return Column(
children: [
Text(pre, style: TextStyle(color: Colors.green[400], fontSize: 16)),
Text('READING AT', style: TextStyle(color: Colors.green[400], fontSize: 16)),
Text(
'${_formatTime(time)}',
style: TextStyle(color: Colors.green[600], fontSize: 24),
)
],
);
}
String _formatTime(int time) {
if (time == 0 || time == null) {
return '00:00';
}
var hours = time ~/ 12;
var minutes = (time % 12) * 5;
return '$hours:$minutes';
}
String _formatIntervalTime(int init, int end) {
var sleepTime = end > init ? end - init : 288 - init + end;
var hours = sleepTime ~/ 12;
var minutes = (sleepTime % 12) * 5;
return '${hours}h${minutes}m';
}
int _generateRandomTime() => Random().nextInt(288);
}