Fixing `GoogleSignIn 0x10461abdc -[GIDSignIn signInWithConfiguration:presentingViewController:hint:additionalScopes:callback:] + 228 (GIDSignIn.m:242)` in Flutter iOS

I recently started using flutterfire_ui's SignInScreen, and it was working great for Android and Web, but not iOS. For some reason, when I clicked on "Sign in with Google", it would crash the app, and throw this debug info:

Exception Type:  EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note:  EXC_CORPSE_NOTIFY
Triggered by Thread:  0

Last Exception Backtrace:
0   CoreFoundation                	       0x111f482f8 __exceptionPreprocess + 160
1   libobjc.A.dylib               	       0x10551131c objc_exception_throw + 56
2   CoreFoundation                	       0x111f48214 -[NSException initWithCoder:] + 0
3   GoogleSignIn                  	       0x10461bf2c -[GIDSignIn signInWithOptions:] + 364 (GIDSignIn.m:529)
4   GoogleSignIn                  	       0x10461abdc -[GIDSignIn signInWithConfiguration:presentingViewController:hint:additionalScopes:callback:] + 228 (GIDSignIn.m:242)
5   Runner                        	       0x10268a718 -[FLTGoogleSignInPlugin handleMethodCall:result:] + 1776
6   Flutter                       	       0x115401520 __45-[FlutterMethodChannel setMethodCallHandler:]_block_invoke + 112
7   Flutter                       	       0x114e8a328 invocation function for block in flutter::PlatformMessageHandlerIos::HandlePlatformMessage(std::__1::unique_ptr<flutter::PlatformMessage, std::__1::default_delete<flutter::PlatformMessage> >) + 108
8   libdispatch.dylib             	       0x10bd41fa4 _dispatch_call_block_and_release + 24
9   libdispatch.dylib             	       0x10bd43768 _dispatch_client_callout + 16
10  libdispatch.dylib             	       0x10bd53018 _dispatch_main_queue_drain + 1220
11  libdispatch.dylib             	       0x10bd52b44 _dispatch_main_queue_callback_4CF + 40
12  CoreFoundation                	       0x111eacb88 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12
13  CoreFoundation                	       0x111ea7318 __CFRunLoopRun + 1956
14  CoreFoundation                	       0x111ea675c CFRunLoopRunSpecific + 584
15  GraphicsServices              	       0x113317c98 GSEventRunModal + 160
16  UIKitCore                     	       0x131ee0b74 -[UIApplication _run] + 868
17  UIKitCore                     	       0x131ee4b1c UIApplicationMain + 124
18  Runner                        	       0x102580cd4 main + 64 (AppDelegate.swift:5)
19  dyld_sim                      	       0x102a29fa0 start_sim + 20
20  dyld                          	       0x102aed08c start + 520
21  ???                           	0xec63800000000000 ???

I tried looking at the repo's GitHub issues but there was nothing.

However, as it crashed in Debug mode, I noticed that some human-readable error message was briefly displayed. So I used QuickTime and started a screen recording. I clicked "Sign in with Google", and crashed the app. Then I watched the recording again. Here's the error it captured:

Unhandled Exception: PlatformException(google_sign_in, Your app is missing support for the following URL schemes: com.googleusercontent.apps....

It looked familiar, so I searched for that string in my project. I also went into the Firebase Console to check. It turns out I was initializing SignInScreen with a Google Client ID specific to Web. Instead, I should have passed the platform specific Google Client ID. So I fixed my initialization from:

final String googleClientId = 'some hard coded string';

SignInScreen(
  auth: auth,
  providerConfigs: [
    GoogleProviderConfiguration(
      clientId: googleClientId,
    )
  ],
  ...

to

final String googleClientId = kIsWeb
    ? [web specific Google Client ID]
    : (Platform.isAndroid
        ? DefaultFirebaseOptions.android.androidClientId
        : DefaultFirebaseOptions.ios.iosClientId);


SignInScreen(
  auth: auth,
  providerConfigs: [
    GoogleProviderConfiguration(
      clientId: googleClientId,
    )
  ],
  ...

And it started working for all platforms. Btw, if the Google Client ID used was the one for Web, why did it even work with Android in the first place? I have no idea.