Says "invalid double" in red screen – Flutter

1. The problem with the provided code is that the calculation of the values of _x1 and _x2 is done inside the build method, which means that it is called every time the widget is built, even if the user has not yet entered any values. This can cause errors or unexpected behavior.
2. So when program runs you are computing x1 and x2 but a,b and c are not entered so null is getting converted to a double (that is the error).
3. To fix this, the calculation of _x1 and _x2 should be moved to a separate function that is only called when the user has entered all the required values.
4. I had made a submit button which will also check if all values are entered or not, and then compute value of x1 and x2 , which i am setting in a state.
5. your formula is also wrong

import ‘dart:math’;
import ‘package:flutter/material.dart’;

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  GlobalKey<FormState> formstate = GlobalKey();
  TextEditingController x2 = TextEditingController();
  TextEditingController x = TextEditingController();
  TextEditingController c = TextEditingController();
  bool showAnswer = false;
  var _x1;
  var _x2;
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('second degree equation'),
          backgroundColor: Colors.black,
        ),
        body: Column(
            mainAxisAlignment: MainAxisAlignment.start,
            mainAxisSize: MainAxisSize.max,
            crossAxisAlignment: CrossAxisAlignment.stretch,
            children: <Widget>[
              Form(
                  key: formstate,
                  child: Column(children: [
                    TextFormField(
                      controller: x2,
                      style: TextStyle(
                          fontSize: 12.0,
                          color: const Color(0xff8b0c0c),
                          fontWeight: FontWeight.w200,
                          fontFamily: "Roboto"),
                      validator: (value) {
                        if (value!.isEmpty) {
                          return 'Please enter a value';
                        }
                        if (double.tryParse(value!) == null) {
                          return 'Please enter a valid number';
                        }
                        return null;
                      },
                    ),
                    Text(
                      "x2+",
                      style: TextStyle(
                          fontSize: 12.0,
                          color: const Color(0xff0e0101),
                          fontWeight: FontWeight.w200,
                          fontFamily: "Roboto"),
                    ),
                    TextFormField(
                      controller: x,
                      style: const TextStyle(
                          fontSize: 12.0,
                          color: const Color(0xffae1a1a),
                          fontWeight: FontWeight.w200,
                          fontFamily: "Roboto"),
                      validator: (value) {
                        if (value!.isEmpty) {
                          return 'Please enter a value';
                        }
                        if (double.tryParse(value!) == null) {
                          return 'Please enter a valid number';
                        }
                        return null;
                      },
                    ),
                    const Text(
                      "x+",
                      style: const TextStyle(
                          fontSize: 12.0,
                          color: const Color(0xff0f0000),
                          fontWeight: FontWeight.w200,
                          fontFamily: "Roboto"),
                    ),
                    TextFormField(
                      controller: c,
                      style: TextStyle(
                          fontSize: 12.0,
                          color: const Color(0xff970c0c),
                          fontWeight: FontWeight.w200,
                          fontFamily: "Roboto"),
                      validator: (value) {
                        if (value!.isEmpty) {
                          return 'Please enter a value';
                        }
                        if (double.tryParse(value!) == null) {
                          return 'Please enter a valid number';
                        }
                        return null;
                      },
                    ),
                  ])),
              ElevatedButton(
                child: Text("submit"),
                onPressed: () {
                  if (formstate.currentState!.validate()) {
                    var a = double.parse(x2.text);
                    var b = double.parse(x.text);
                    var c = double.parse(this.c.text);
                    var temp_x1 = ((-1 * b) + sqrt((b * b) - (4 * a * c))) / (2 * a);
                    var temp_x2 = ((-1 * b) - sqrt((b * b) - (4 * a * c))) / (2 * a);
                     setState(() {
                       showAnswer=true;
                       _x1=temp_x1;
                       _x2=temp_x2;
                     });
                     print(_x1);
                     print(_x2);
                  }
                }
              ),
              if(showAnswer)
              Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  Text('$_x1'+"  "+'$_x2')
                ],
              )

            ]
        ),
    );
  }

              
  
  }

5 and 3 are values of x1 and x2