The supplied auth credential is malformed or has expired. - Flutter Facebook Login

I am trying to implement FacebookLogin in my flutter app using Firebase Realtime Database. I am able to login to my app. However, when I try to fetch something from the Database using the FB's auth token, I get the following error:

 E/flutter (30549): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: [firebase_auth/invalid-credential] The supplied auth credential is malformed or has expired. [ Unsuccessful debug_token response from Facebook: {"error":{"message":"Invalid OAuth access token signature.","type":"OAuthException","code":190,"fbtrace_id":"AwuVQUt6tCG1go"}} ]

Here's my facebook login code:

Future signInFB() async {
      var facebookLogin = FacebookLogin();
    var facebookLoginResult =
        await facebookLogin.logIn(['email']);
     switch (facebookLoginResult.status) {
      case FacebookLoginStatus.error:
        break;
      case FacebookLoginStatus.cancelledByUser:
        print("CancelledByUser");           
        break;
      case FacebookLoginStatus.loggedIn:
        print("LoggedIn");
        var graphResponse = await http.get(
            'https://graph.facebook.com/v2.12/me?fields=name,first_name,last_name,email,picture.height(200)&access_token=${facebookLoginResult
                .accessToken.token}');
          fbToken = facebookLoginResult.accessToken.token.toString();
    var profile = json.decode(graphResponse.body);
    final FirebaseAuth _auth = FirebaseAuth.instance;
    print('Token is ' + fbToken); 
    final OAuthCredential credential =  FacebookAuthProvider.credential(fbToken); 
    final User user = (await _auth.signInWithCredential(credential)).user;
    assert(!user.isAnonymous);
    assert(await user.getIdToken() != null);
     _token = await user.getIdToken();
     _userId = user.uid;
    print('Profile is ' + profile.toString());
    break;
  }
  }