NoSuchMethodError: The getter 'isEmpty' was called on null

I was checking back the flutter_firebase_ui and try to run their sample but I could no longer get it to work even on email/password sign-in option. By the time I clicked the Next button (screenshot), the error below comes out in the terminal.

Running pod install...
Starting Xcode build...
Xcode build done.
Falling back to loading access token from NSUserDefaults because of simulator bug
Falling back to storing access token in NSUserDefaults because of simulator bug
Falling back to loading access token from NSUserDefaults because of simulator bug
Falling back to storing access token in NSUserDefaults because of simulator bug
Falling back to loading access token from NSUserDefaults because of simulator bug
Falling back to storing access token in NSUserDefaults because of simulator bug
FBSDKLog: Missing [FBSDKAppEvents appID] for [FBSDKAppEvents publishInstall:]
FBSDKLog: Missing [FBSDKAppEvents appEventsState.appID] for [FBSDKAppEvents flushOnMainQueue:]
FBSDKLog: Missing [FBSDKAppEvents appEventsState.appID] for [FBSDKAppEvents flushOnMainQueue:]
 [1.1 EBD4E4A9-47B1-4B73-8ACA-F43768B85BF1 192.168.1.7.51464<->216.58.220.202:443]
 Connected Path: satisfied (Path is satisfied), interface: en0
 Duration: 0.488s, DNS @0.010s took 0.064s, TCP @0.077s took 0.089s, TLS took 0.202s
 bytes in/out: 3308/1072, packets in/out: 9/10, rtt: 0.085s, retransmitted packets: 0, out-of-order packets: 0
 [2.1 9C251829-7C8E-4A4A-8F82-D1C927E371F7 192.168.1.7.51465<->31.13.95.8:443]
 Connected Path: satisfied (Path is satisfied), interface: en0
 Duration: 0.488s, DNS @0.001s took 0.090s, TCP @0.092s took 0.043s, TLS took 0.105s
 bytes in/out: 4126/999, packets in/out: 8/8, rtt: 0.041s, retransmitted packets: 0, out-of-order packets: 0
5.4.0 - [Firebase/Core][I-COR000003] The default Firebase app has not yet been configured. Add `[FIRApp configure];` (`FirebaseApp.configure()` in Swift) to your application initialization. Read more: https://goo.gl/ctyzm8.
Syncing files to device iPhone X...
5.4.0 - [Firebase/Analytics][I-ACS023007] Firebase Analytics v.50001000 started
5.4.0 - [Firebase/Analytics][I-ACS023008] To enable debug logging set the following application argument: -FIRAnalyticsDebugEnabled (see http://goo.gl/RfcP7r)
 [3.1 42F1BD47-5FCD-4865-B579-6A1D3E504F3B 192.168.1.7.51496<->172.217.161.138:443]
 Connected Path: satisfied (Path is satisfied), interface: en0
 Duration: 0.813s, DNS @0.001s took 0.015s, TCP @0.019s took 0.093s, TLS took 0.206s
 bytes in/out: 3704/1032, packets in/out: 9/10, rtt: 0.094s, retransmitted packets: 0, out-of-order packets: 0
flutter: null
flutter: NoSuchMethodError: The getter 'isEmpty' was called on null.
Receiver: null
Tried calling: isEmpty

Anyone encountered like this and fixed this kind of error?