Access the document object from Firebase database (Firestore)

I use FirebaseX plugin and its method fetchDocumentInFirestoreCollection to access a document from my Firebase database (the document does exist). It successfully passes the success callback function but I am never able to access the document object. I do not know how to do. Here are my two used approaches to access:

 await this.firebase.fetchDocumentInFirestoreCollection(
          someDocID,
          'someCollection',
          () => {
            console.log('fetchFirestoreCollection successfully'); // this can be printed
          },
          error => {
            console.error('error in fetchFirestoreCollection', error);
          }
        ).then(
          doc => {
            // Not enter this block ever
            console.log(doc);
          }
        );
// doc is null all the way
 const doc = await this.firebase.fetchDocumentInFirestoreCollection(
          someDocID,
          'someCollection',
          () => {
            console.log('fetchFirestoreCollection successfully'); // this can be printed
          },
          error => {
            console.error('error in fetchFirestoreCollection', error);
          }
        );

But all of these failed to access the returned document. How should I do?

ionic info:

Ionic:
   Ionic CLI                     : 5.4.16
   Ionic Framework               : @ionic/angular 5.1.0
   @angular-devkit/build-angular : 0.901.4
   @angular-devkit/schematics    : 9.1.4
   @angular/cli                  : 9.1.4
   @ionic/angular-toolkit        : 2.2.0

Cordova:
   Cordova CLI       : 9.0.0 (cordova-lib@9.0.1)
   Cordova Platforms : android 8.1.0
   Cordova Plugins   : cordova-plugin-ionic-keyboard 2.2.0, cordova-plugin-ionic-webview 4.2.1, (and 6 other plugins)

cordova plugin:

cordova-plugin-advanced-http 2.4.1 "Advanced HTTP plugin"
cordova-plugin-androidx 1.0.2 "cordova-plugin-androidx"
cordova-plugin-androidx-adapter 1.1.0 "cordova-plugin-androidx-adapter"
cordova-plugin-device 2.0.2 "Device"
cordova-plugin-enable-multidex 0.2.0 "Enable Multidex"
cordova-plugin-file 6.0.2 "File"
cordova-plugin-firebase-authentication 3.3.0 "cordova-plugin-firebase-authentication"
cordova-plugin-firebasex 9.1.1 "Google Firebase Plugin"
cordova-plugin-ionic-keyboard 2.2.0 "cordova-plugin-ionic-keyboard"
cordova-plugin-ionic-webview 4.2.1 "cordova-plugin-ionic-webview"
cordova-plugin-splashscreen 5.0.2 "Splashscreen"
cordova-plugin-statusbar 2.4.2 "StatusBar"
cordova-plugin-whitelist 1.3.3 "Whitelist"
cordova-sqlite-storage 5.0.0 "Cordova sqlite storage plugin - cordova-sqlite-storage plugin version"
cordova-support-android-plugin 1.0.2 "cordova-support-android-plugin"
cordova-support-google-services 1.4.0 "cordova-support-google-services"

In the @ionic-native/firebase-x/ngx/index.d.ts, change line 437 from

fetchDocumentInFirestoreCollection(documentId: string, collection: string, success: () => void, error: (err: string) => void): Promise<any>;

to

fetchDocumentInFirestoreCollection(documentId: string, collection: string, success: (value: object) => void, error: (err: string) => void): Promise<any>;