Null exception when using filter on aggregation helper ElasticSearch

I created an AggregationsHelper object and I called .Filter method on it

 var aggr = new AggregationsHelper();
 aggr .Filter("test");

I get a null exception when calling the .Filter function. Do I have to initiate the object somehow?

EDIT:

As pointed out by @Val, I have to pass a dictionary to the object via the constructor

My real question is how to mock such complex ElasticSearch aggregation query:

    var res = ElasticClient.Search<DataRecord>(s => s
            .SearchType(SearchType.Count)
            .Aggregations(a => a.Filter(
                "histIdFilter",
                f => f.Filter(
                    f2 => f2.Term(
                        t => t.HistoryId,
                        groupId))
            .Aggregations(ag => ag.Filter("timeRangeFilter", fg => fg.Filter(fg2 => fg2.Range(i => i.OnField(b => b.DateTime))))))));

    // Applying filter
    var ah = res.Aggs;
    var histIdAgg = ah.Filter("histIdFilter");
    var timeRangeAgg = histIdAgg.Filter("timeRangeFilter");

I successfully managed to mock the .Aggr property of the query response, using

datResp.SetupGet(x => x.Aggs).Returns(ah.Object);

but when I call .Filter on the ah object I get a null exception like if the internal dictionary was not set for some reason