How to show loading spinner in the button when sign in using firebase

i have create a variable "isLoading" for calling the loader. the actual problem is when i hit simply in the "signIn" button without give any value in the test field, it will start loading for delay time how can i stop this and also catch the error when hit the "signIn" button without given any value.... this is my Repo: food delivery app

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import '../../config/constants.dart';

class LoginScreen extends StatefulWidget {
  const LoginScreen({Key? key}) : super(key: key);

  @override
  State<LoginScreen> createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  final emailController = TextEditingController();
  final passwordController = TextEditingController();
  @override
  void dispose() {
    emailController.dispose();
    passwordController.dispose();

    super.dispose();
  }

  bool isLoading = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        padding: const EdgeInsets.all(20),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          crossAxisAlignment: CrossAxisAlignment.center,
          children: [
            const Text('login'),
            htspace20,
            TextField(
              controller: emailController,
              decoration: const InputDecoration(
                border: OutlineInputBorder(),
                label: Text('Email'),
              ),
            ),
            htspace20,
            TextField(
              controller: passwordController,
              decoration: const InputDecoration(
                border: OutlineInputBorder(),
                label: Text('Password'),
              ),
            ),
            htspace40,
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: [
                TextButton(onPressed: () {}, child: const Text('Sign up'))
              ],
            ),
            SizedBox(
              width: double.maxFinite,
              height: 40,
              child: ElevatedButton(
                  style: ButtonStyle(
                    elevation: MaterialStateProperty.all(0),
                  ),
                  child: isLoading
                      ? const SizedBox(
                          height: 30,
                          width: 30,
                          child: CircularProgressIndicator(
                              strokeWidth: 3, color: Colors.white),
                        )
                      : const Text('Sign in'),
                  onPressed: () {
                    setState(() {
                      isLoading = true;
                    });
                    signIn();
                    Future.delayed(const Duration(seconds: 3), () {
                      setState(() {
                        if (emailController.text != " " ||
                            passwordController.text != " ") {
                          isLoading = false;
                        } else {
                          isLoading = true;
                        }
                      });
                    });
                  }),
            )
          ],
        ),
      ),
    );
  }

  Future signIn() async {
    try {
      await FirebaseAuth.instance.signInWithEmailAndPassword(
        email: emailController.text.trim(),
        password: passwordController.text.trim(),
      );
    } catch (e) {
      print('Email or Password Incorrect');
    }
  }
}