How to fix auth/account-exists-with-different-credential in Firebase

import { signInWithRedirect, getRedirectResult, GoogleAuthProvider } from 'firebase/auth';

try {
  const result = await getRedirectResult(auth);
  // Sign in successful
} catch (error) {
  if (error.code === 'auth/account-exists-with-different-credential') {
    // Extract the pending credential from the error
    const pendingCredential = GoogleAuthProvider.credentialFromError(error);

    // Get the email to display to user
    const email = error.customData?.email;

    // Prompt user to sign in with their existing provider
    console.log('Account exists with different credential for:', email);
    // Store pendingCredential for linking in next step
  }
}

try {
  final result = await FirebaseAuth.instance.signInWithPopup(googleProvider);
} on FirebaseAuthException catch (e) {
  if (e.code == 'account-exists-with-different-credential') {
    // Get the email associated with the account
    final email = e.email;

    // Get list of sign-in methods for this email
    final methods = await FirebaseAuth.instance.fetchSignInMethodsForEmail(email!);
    print('Sign-in methods available: $methods');
  }
}

const email = error.customData?.email;
const methods = await auth.fetchSignInMethodsForEmail(email);

// Example: methods might be ['facebook.com', 'password']
alert(`This email is already registered with ${methods.join(', ')}. Please sign in with that provider first.`);

// Redirect user to sign in with their existing provider
// Example: sign in with Facebook if that's their existing provider
const result = await signInWithPopup(auth, facebookProvider);

// After user successfully signs in with their existing provider
try {
  // Link the pending credential (from the original sign-in attempt) to the current user
  await linkWithCredential(auth.currentUser, pendingCredential);
  console.log('Account linked successfully!');
} catch (error) {
  console.error('Failed to link accounts:', error);
}

try {
  // After signing in with existing provider, get user and link new provider
  final facebookCredential = await FirebaseAuth.instance.signInWithPopup(facebookProvider);
  await auth.currentUser?.linkWithCredential(googleAuthCredential);
  print('Accounts linked!');
} catch (e) {
  print('Link failed: ${e.message}');
}