I got these errors on terminal

The getter 'pi' was called on null. Receiver: null Tried calling: pi

I tried doing this "pi != null", but no luck. Likewise, changed "pi" with "io" to see if imported the package "dart:io" works, but it doesn't, the second thing that I do was delete "pi", to just math!, but again no luck.

This is my code:

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'dart:core';
import 'dart:math';
import 'dart:io';
class CountDownTimer extends StatefulWidget {
  const CountDownTimer({Key? key}) : super(key: key);

  @override
  State<CountDownTimer> createState() => _CountDownTimerState();
}

class _CountDownTimerState extends State<CountDownTimer> with TickerProviderStateMixin {
  late AnimationController controller;

  String get timerString {
    Duration duration = controller.duration! * controller.value;
    return '${duration.inMinutes}:${(duration.inSeconds % 60).toString().padLeft(2, '0')}';
  }

  @override
  void initState() {
    super.initState();
    controller = AnimationController(
      vsync: this,
      duration: Duration(seconds: 5),
    );
  }

  @override
  Widget build(BuildContext context) {
    ThemeData themeData = Theme.of(context);
    return Scaffold(
      backgroundColor: Colors.white10,
      body:
      AnimatedBuilder(
          animation: controller,
          builder: (context, child) {
            return Stack(
              children: <Widget>[
                Align(
                  alignment: Alignment.bottomCenter,
                  child: Container(
                    color: Colors.amber,
                    height:
                    controller.value * MediaQuery.of(context).size.height,
                  ),
                ),
                Padding(
                  padding: EdgeInsets.all(8.0),
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.spaceBetween,
                    children: <Widget>[
                      Expanded(
                        child: Align(
                          alignment: FractionalOffset.center,
                          child: AspectRatio(
                            aspectRatio: 1.0,
                            child: Stack(
                              children: <Widget>[
                                Positioned.fill(
                                  child: CustomPaint(
                                      painter: CustomTimerPainter(
                                        animation: controller,
                                        backgroundColor: Colors.white,
                                        color: themeData.indicatorColor,
                                      )),
                                ),
                                Align(
                                  alignment: FractionalOffset.center,
                                  child: Column(
                                    mainAxisAlignment:
                                    MainAxisAlignment.spaceEvenly,
                                    crossAxisAlignment:
                                    CrossAxisAlignment.center,
                                    children: <Widget>[
                                      Text(
                                        "Count Down Timer",
                                        style: TextStyle(
                                            fontSize: 20.0,
                                            color: Colors.white),
                                      ),
                                      Text(
                                        timerString,
                                        style: TextStyle(
                                            fontSize: 112.0,
                                            color: Colors.white),
                                      ),
                                    ],
                                  ),
                                ),
                              ],
                            ),
                          ),
                        ),
                      ),
                      AnimatedBuilder(
                          animation: controller,
                          builder: (context, child) {
                            return FloatingActionButton.extended(
                                onPressed: () {
                                  if (controller.isAnimating)
                                    controller.stop();
                                  else {
                                    controller.reverse(
                                        from: controller.value == 0.0
                                            ? 1.0
                                            : controller.value);
                                  }
                                },
                                icon: Icon(controller.isAnimating
                                    ? Icons.pause
                                    : Icons.play_arrow),
                                label: Text(
                                    controller.isAnimating ? "Pause" : "Play"));
                          }),
                    ],
                  ),
                ),
              ],
            );
          }),
    );
  }
}

class CustomTimerPainter extends CustomPainter {
  CustomTimerPainter({
    required this.animation,
    required this.backgroundColor,
    required this.color,
  }) : super(repaint: animation);

  final Animation<double> animation;
  final Color backgroundColor, color;

  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
      ..color = backgroundColor
      ..strokeWidth = 10.0
      ..strokeCap = StrokeCap.butt
      ..style = PaintingStyle.stroke;

    canvas.drawCircle(size.center(Offset.zero), size.width / 2.0, paint);
    paint.color = color;
    var math;
    double progress = (1.0 - animation.value) * 2 * math.pi;
    canvas.drawArc(Offset.zero & size, math.pi * 1.5, -progress, false, paint);
  }
  @override
  bool shouldRepaint( CustomTimerPainter old) {
    return animation.value != old.animation.value ||
        color != old.color ||
        backgroundColor != old.backgroundColor;
  }
}

How to solve this issue? Thank you in advance.

CodePudding user response：

The issue is with this line

    var math;

You have defined a variable called math, but haven't assigned it to anything. Since math is null, you get the error 'the getter 'pi' was called on null'.

You need to delete this line.

Your code will now work if you do

import 'dart:math' as math;

However, the math prefix is not necessary, so you could also do

import 'dart:math;

And use pi, instead of math.pi