How to unit test the firebase connection when code is written in Bloc-pattern?

I have written my code n Bloc pattern based on this article. In my code, I need to connect to a real-time database and fetch some data from it. But unfortunately, I can't write a bug-free unit test for it.

This is my ‍‍Res/filename_api_provider(based on the above link) class that provide the connection criteria to firebase:

class filenameApiProvider {
    var response; 
    Future<fileModel> fetchList() async {
        DatabaseReference ref = FirebaseDatabase.instance.reference();
        await ref.child("exercises").once().then((DataSnapshot snap) {
            response = snap.value;
        });

    if (response != null) {
        return fileModel.fromJsonWorkouts(response);
    } else {
        throw Exception('Failed to load workout library api’s');
    }
}

and if the response comes from the Firebase server I pass it to a model class to fill some list and show them in UI I tried to write a test for this class and this is my attempt:

void main() {
    test('fetch data', () async {
        var obj = new filenameApiProvider();
        fileModel result = await obj.fetchWorkoutList();
         expect(result, isNotNull);
    });
 }

But this is an error I receive after running test:

MissingPluginException(No implementation found for method Query#observe on channel plugins.flutter.io/firebase_database)

package:flutter/src/services/platform_channel.dart 300:7  MethodChannel.invokeMethod
===== asynchronous gap ===========================
dart:async                                                _AsyncAwaitCompleter.completeError
package:flutter/src/services/platform_channel.dart        MethodChannel.invokeMethod
===== asynchronous gap ===========================
dart:async                                                _asyncThenWrapperHelper
package:flutter/src/services/platform_channel.dart        MethodChannel.invokeMethod
package:firebase_database/src/query.dart 53:38            Query._observe.<fn>
dart:async                                                Stream.first
package:firebase_database/src/query.dart 90:55            Query.once