Revoking 3pid invites leads to "undefined" in chat

[turt2live]

Description

as per https://github.com/vector-im/riot-web/issues/5487#issuecomment-341137283

The event looks something like this (excess content removed):

{
  "origin_server_ts": 1509548272512,
  "sender": "@patroniser:matrix.org",
  "event_id": "$15095482722600638uZWpv:matrix.org",
  "unsigned": {
    "prev_content": {
      "display_name": "email...@address...",
    },
    "prev_sender": "@patroniser:matrix.org",
    "replaces_state": "$1507650532380634OzePO:matrix.org",
    "age": 23729
  },
  "state_key": "some_long_string",
  "content": {},
  "type": "m.room.third_party_invite",
  "room_id": "!MhJgddNhalZCKBgDvw:matrix.org"
}

(key information removed)

Version information

• Platform: web (in-browser)
• Browser: Chrome 62
• OS: Windows 10
• URL: riot.im/develop