Reading properties from nested _aggs inside scripted metric aggregation

Elastic Stack Elasticsearch
1

Hello.

I was handling game log, and I was trying to check every user's gold at the end of each day with aggs and scripted metrics. My english is bad, so I'll jump right to the code

GET /game/log/_search
{ 
  "query": {
   "match_all" : {}
  },"size": 0,
 
  "aggs": {
    "human": {
      "terms": {
        "field": "playerid",
        "size": 10000000
      }, 
  "aggs": {
    "histo_bro": {
      "date_histogram": {
        "field": "@timestamp",
        "interval": "day"
      },
      "aggs": {
        "lastgold": {
          "scripted_metric": {
            "init_script": "_agg['time'] = 0",
            "map_script": "if(doc['@timestamp'].value > _agg['time']) {_agg['time'] = doc['@timestamp'].value ; _agg['gold'] = doc['gold'].value}",
            "combine_script": "return _agg"
//            ,"reduce_script": "res=0; for(t in _aggs){if(t==null) return 100};return res"
//            ,"reduce_script": "chktime = 0 ; res = 0; for(t in aggs){if(t.time > chktime){chktime = t.time ; res = t.gold}}; return res"
            ,"reduce_script": "return _aggs"
          }
        }
      }
    }
  }
}

the script above will show me something like this whole lot of time:

    {
      "key": //userid,
      "doc_count": 107,
      "histo_bro": {
        "buckets": [
          {
            "key_as_string": "2016-07-12T00:00:00.000Z",
            "key": 1468281600000,
            "doc_count": 107,
            "lastgold": {
              "value": [
                {
                  "time": 1468287233096,
                  "gold": 105414
                },
                {
                  "time": 1468287554348,
                  "gold": 281015
                },
                {
                  "time": 1468286842445,
                  "gold": 165175
                },
                {
                  "time": 1468287473573,
                  "gold": 87537
                },
                {
                  "time": 1468287039060,
                  "gold": 75946
                },
                {
                  "time": 1468287234215,
                  "gold": 106953
                },
                {
                  "time": 1468287027673,
                  "gold": 68126
                },
                {
                  "time": 1468287473573,
                  "gold": 87537
                },
                {
                  "time": 1468287553047,
                  "gold": 277214
                },
                {
                  "time": 1468287470048,
                  "gold": 80241
                }
              ]
            }
          }
        ]
      }
    },

enabling first line of reduce_script (and disabling third) will return :

aggregations": {
    "human": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": //userid,
          "doc_count": 134,
          "histo_bro": {
            "buckets": [
              {
                "key_as_string": "2016-07-12T00:00:00.000Z",
                "key": 1468281600000,
                "doc_count": 134,
                "lastgold": {
                  "value": 0
                }
              }
            ]
          }
        },

which means t is not null.

but enabling second line of reduce script will throw me exception :

 "caused_by": {
     "type": "null_pointer_exception",
     "reason": "Cannot get property 'time' on null object"
  }

Why does this happen? How can I read 'time' and 'gold' from say like _aggs[0] ?