[Flutter] How to go with this error "The method '/' was called on null"

I was just doing a BMI Calculator with Flutter but end up having this error but when I look at all my codes I could not find the error.

Here is my calculator_brain.dart file where reside all my conditions:

import 'dart:math';

class CalculatorBrain {
CalculatorBrain({this.height, this.weight});
int height;
int weight;

double _bmi;

String calculateBMI() {
_bmi = weight / pow(height / 100, 2);
return _bmi.toStringAsFixed(1);
}

String getResult() {
if (_bmi >= 25) {
return 'OverWeight';
} else if (_bmi >= 18.5) {
return 'Normal';
} else {
return 'UnderWeight';
}
}

String getInterpretation() {
if (_bmi >= 25) {
return 'You have a higher than the normal body weight! You need to exercice more...';
} else if (_bmi >= 18.5) {
return 'You\'ve got a normal body weight! Good Job!';
} else {
return 'you have a lower than the normal body weight! You\'d be eating more...';
}
}
}


my results.dart to display everything. it is even the screen which is not showing for the error

import 'package:bmi_calculator/constants.dart';
import 'package:bmi_calculator/components/reusable_card.dart';
import 'package:flutter/material.dart';
import '../components/bottom_button.dart';

class Results extends StatelessWidget {
Results(
{@required this.bmiResult,
@required this.resultText,
@required this.interpretation});
final String bmiResult;
final String resultText;
final String interpretation;
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('BMI CALCULATOR'),
),
body: Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
crossAxisAlignment: CrossAxisAlignment.stretch,
children: <Widget>[
Expanded(
child: Container(
padding: EdgeInsets.all(15.0),
alignment: Alignment.bottomLeft,
child: Text(
'Your Result',
style: kTitleTextStyle,
),
),
),
Expanded(
flex: 5,
child: ReusableCard(
colour: kActiveCardColor,
cardChild: Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
crossAxisAlignment: CrossAxisAlignment.center,
children: <Widget>[
Text(
resultText.toUpperCase(),
style: kResultTextStyle,
),
Text(
bmiResult,
style: kBMITextstyle,
),
Text(
interpretation,
style: kBodyTextStyle,
textAlign: TextAlign.center,
),
],
),
),
),
Expanded(
child: BottomButton(
buttonTitle: 'RE-CALCULATE',
onTap: () {
Navigator.pop(context);
},
),
)
],
),
);
}
}


Lastly, the object that I'm creating for the calculator and the navigator:

BottomButton(
buttonTitle: 'CALCULATE',
onTap: () {
CalculatorBrain calculationBrain = CalculatorBrain();
Navigator.push(
context,
MaterialPageRoute(
builder: (context) {
return Results(
bmiResult: calculationBrain.calculateBMI(),
resultText: calculationBrain.getResult(),
interpretation: calculationBrain.getInterpretation(),
);
},
),
);
},
),


The complete error I am receiving is "The following NoSuchMethodError was thrown building Builder(dirty)":

The method '/' was called on null.
Receiver: null
Tried calling: /(100)

The relevant error-causing widget was:
MaterialApp file:///Users/macair13/AndroidStudioProjects/bmi-calculator-flutter/lib/main.dart:9:12
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1 CalculatorBrain.calculateBMI (package:bmi_calculator/calculator_brain.dart:11:32)
#2 _InputPageState.build.<anonymous closure>.<anonymous closure> (package:bmi_calculator/screens/input_page.dart:222:51)
#3 MaterialPageRoute.buildPage (package:flutter/src/material/page.dart:87:27)
#4 _ModalScopeState.build.<anonymous closure> (package:flutter/src/widgets/routes.dart:710:43)

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