Built a timer in flutter but if one textfield input is null then the timer wont start

?? is use for checking null in your case it's empty string so please check isNotEmpty.

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:flutter_plugg/main.dart';


class TimerView extends StatefulWidget {
  @override
  _TimerViewState createState() => _TimerViewState();
}

class _TimerViewState extends State<TimerView> with TickerProviderStateMixin {
  TextEditingController _minuteController = TextEditingController();
  TextEditingController _hourController = TextEditingController();
  TextEditingController _secondController = TextEditingController();

  AnimationController _controller;

  Animation<int> _animation;

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  void _pauseTimer() {
    _controller.stop();
  }

  void _stopTimer() {
    _controller.reset();
  }

  void _startTimer() {
    int timerInput = int.parse(_hourController.text.isNotEmpty ? _hourController.text : '0') * 3600 +
        int.parse(_minuteController.text.isNotEmpty ? _minuteController.text : '0') * 60 +
        int.parse(_secondController.text.isNotEmpty ? _secondController.text : '0');
    _controller = AnimationController(
      vsync: this,
      duration: Duration(seconds: timerInput),
    );

    _animation = StepTween(begin: timerInput, end: 0).animate(_controller)
      ..addListener(() {
        _setTimerValues();
      });
    _controller.forward();
  }

  void _setTimerValues() {
    setState(() {
      print(_animation.value);
      _hourController.text = (_animation.value ~/ 3600).toString();

      _minuteController.text = ((_animation.value % 3600) ~/ 60).toString();

      _secondController.text = (_animation.value % 60).toString();
    });
  }

  Widget _textInput({TextEditingController controller}) {
    return Container(
      width: 100,
      height: 60,
      child: TextField(
        showCursor: false,
        enableInteractiveSelection: false,
        controller: controller,
        style: TextStyle(
          fontSize: 60,
        ),
        inputFormatters: [FilteringTextInputFormatter.digitsOnly],
        keyboardType: TextInputType.number,
        textAlign: TextAlign.center,
        maxLength: 2,
        decoration: InputDecoration(
          contentPadding: EdgeInsets.zero,
          hintStyle: TextStyle(color: Colors.grey),
          counterText: "",
          hintMaxLines: 1,
          hintText: "00",
          border: InputBorder.none,
        ),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      drawer: PersistentDrawer(currentPage: "Timer"),
      appBar: AppBar(
        title: Text("Timer"),
      ),
      body: Column(
        children: [
          SizedBox(height: 80),
          Row(
            crossAxisAlignment: CrossAxisAlignment.end,
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              _textInput(controller: _hourController),
              Text("h", style: TextStyle(fontSize: 24)),
              _textInput(controller: _minuteController),
              Text("m", style: TextStyle(fontSize: 24)),
              _textInput(controller: _secondController),
              Text("s", style: TextStyle(fontSize: 24))
            ],
          ),
          SizedBox(
            height: 60,
          ),
          Row(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Padding(
                padding: const EdgeInsets.only(right: 15),
                child: ElevatedButton(
                  onPressed: () {
                    _controller?.isAnimating ?? false ? _pauseTimer() : _startTimer();
                    setState(() {});
                  },
                  child: _controller?.isAnimating ?? false ? Text("Pause Timer") : Text("Start Timer"),
                ),
              ),
              Padding(
                padding: const EdgeInsets.only(left: 15),
                child: ElevatedButton(
                  onPressed: _stopTimer,
                  child: Text("Stop Timer"),
                ),
              )
            ],
          ),
        ],
      ),
    );
  }
}

you have to check of the the value is empty or null.

for that create a function _format which takes a value a string value. it checks if the value is valid and return a valid output.

String _format(value) {
    if (value != null && value.isNotEmpty) {
        return value;
    }
    return "0";
}

and call the function inside your _startTimer function.

void _startTimer() {
    int timerInput = int.parse(_format(_hourController.text)) * 3600 +
        int.parse(_format(_minuteController.text)) * 60 +
        int.parse(_format(_secondController.text));
    _controller = AnimationController(
        vsync: this,
        duration: Duration(seconds: timerInput),
    );

    _animation = StepTween(begin: timerInput, end: 0).animate(_controller)
        ..addListener(() {
            _setTimerValues();
        });
    _controller.forward();
}