Error when rendering Single Message view for message in deleted input

An error occurs when the following GET request is executed when viewing the Single Message page for a message that was received through an input which is now deleted.

`GET http://localhost:9000/api/system/inputs/{input-id}`


## Expected Behavior
I am not sure what is loading this Input details resource on this page, but it does not work when the input is deleted.

## Possible Solution

## Steps to Reproduce (for bugs)
1. Receive a message in Graylog.
2. Delete the input that the message was received through.
3. Go to the search page and find the message.
4. Click the message to go to the Message Details page.

## Context

## Your Environment

* Graylog Version: Graylog 3.1.0-beta.3-SNAPSHOT (master)