500 internal server error while invoking a non-blocking action (very rare situation)

## Environment details:

* local deployment: testing environment on SoftLayer
* version of docker: 1.12.0

## Steps to reproduce the issue:

Hard to reproduce. We saw that in 1/100k requests during a heavy load scenario on a 10 invoker environment.

## Provide the expected results and outputs:

response code `202` is expected.

## Provide the actual results and outputs:

```
{
"error": "There was an internal server error.",
"code": 173953
}
```

```
June 13th 2017, 23:25:51.348 [2017-06-13T23:25:51.348Z] [INFO] [#tid_173953] [Invoker] recording the activation result to the data store invoker5
June 13th 2017, 23:25:51.348 [2017-06-13T23:25:51.348Z] [INFO] [#tid_173953] [Invoker] running result: ok [marker:invoker_activationRun_finish:29:16] invoker5
June 13th 2017, 23:25:51.348 [2017-06-13T23:25:51.348Z] [INFO] [#tid_173953] [ContainerPool] putBack returning container 4967 delete = false completed = 18578 slack = 15 maxActive = 16 activeCount = 1 startingCounter = 0 invoker5
June 13th 2017, 23:25:51.339 [2017-06-13T23:25:51.339Z] [ERROR] [#tid_173953] [ActionsApi] [POST] action activation failed: null controller
June 13th 2017, 23:25:51.339 [2017-06-13T23:25:51.339Z] [INFO] [#tid_173953] [BasicHttpService] [marker:http_post.500_count:20:20] controller
June 13th 2017, 23:25:51.331 [2017-06-13T23:25:51.331Z] [INFO] [#tid_173953] [Invoker] sending arguments to XXXXX/XXXXXX@0.0.1 container [wsk5_6395_warmJsContainer_20170613T232224627Z] [4b80c92d2982927aaf50a718b225d3622b8851bdf4114932cea63c34f115754a] [192.168.0.2:8080] [marker:invoker_activationRun_start:13]
```