Angular config service results in errors in tests

I've got a config init service which I run as soon as app starts(to import server URIs from .json).

@Injectable({
    providedIn: 'root'
})
export class ConfigInitService {

    static settings: UriConfig;

    load() {
    }

    constructor(
        private http: HttpClient
    ) {
        this.http.get('assets/config/uris.json').subscribe((file: UriConfig) => {
            ConfigInitService.settings = file;
        });
    }
}

and this is my app.module

providers: [
        ConfigInitService,
        {
            provide: APP_INITIALIZER,
            useFactory: () => initializeApp,
            deps: [ConfigInitService],
            multi: true
        },
...
})
export class AppModule {
}

export function initializeApp(uriConfig: ConfigInitService) {
    return () => uriConfig.load();
}

The problem is that when I run tests they have errors (sometimes not) that it cannot read property socket of undefined. Socket is a property of that json file. Meanwhile app works I cannot make my tests work. The trick is that this error appears dynamically over tests runs. That means that any time I run my tests the component or components that emit this error change. Moreover, sometimes error is printed out in components that even do not use this service or its dependent service (that service just has functions that returns different strings from the loaded config).

Moreover, I've got another application with identical init service and those tests work fine.