方法“ <”在null上被调用

Question

我尝试运行我的应用程序，并收到此错误消息。

方法'<'在null上被调用。接收方：null尝试调用： <（18.5）

相关的引起错误的小部件是：

MaterialApp  lib\main.dart:11 When the exception was thrown, this was the stack
#0      Object.noSuchMethod  (dart:core-patch/object_patch.dart:53:5)
#1      CalculateBMI.getBMI  package:health_calculator/bmi_calc.dart:11
#2      _ThirdScreenState.build.<anonymous closure>.<anonymous closure>  package:health_calculator/third_screen.dart:175
#3      MaterialPageRoute.buildPage  package:flutter/…/material/page.dart:87
#4      _ModalScopeState.build.<anonymous closure>  package:flutter/…/widgets/routes.dart:710 ...

这是我的代码：

String getBMI() {
  if (BMI < 18.5) {
    return 'Underweight';
  } else if (BMI >= 18.5 || BMI <= 24.9) {
    return 'Healthy Weight';
  } else if (BMI >= 25.0 || BMI <= 29.9) {
    return 'Overweight';
  } else {
    return 'Obese';
  }
}

String getRange() {
  var BMI = weight / pow(height / 100, 2);
  if (BMI < 18.5) {
    return '(BMI less than 18.5)';
  } else if (BMI >= 18.5 || BMI <= 24.9) {
    return '(BMI 18.5 to 24.9)';
  } else if (BMI >= 25.0 || BMI <= 29.9) {
    return '(BMI 25 to 30)';
  } else {
    return '(BMI 30+)';
  }
}

returnBMI() {
  BMI = weight / pow(height / 100, 2);
  return BMI;
}