navigating outline via arrowkeys crashes vscodevim

To Reproduce
Steps to reproduce the behavior:

1. Use File: Focus on Outline to focus the outline
2. Navigate with the arrowkeys to any element
3. jump to it via ENTER
4. VSCodeVim will have crashed (none of the bindings work anymore, e.g. j/k). The devtools show the following error:

   ERR Converting circular structure to JSON: TypeError: Converting circular structure to JSON
    at JSON.stringify (<anonymous>)
    at Function.e.serializeReplyOK (file:///usr/lib/code/out/vs/workbench/workbench.main.js:3240:532)
    at file:///usr/lib/code/out/vs/workbench/workbench.main.js:3234:276
    at n.Class.derive._creator.then (file:///usr/lib/code/out/vs/workbench/workbench.main.js:161:366)
    at e._receiveRequest (file:///usr/lib/code/out/vs/workbench/workbench.main.js:3234:176)
    at e._receiveOneMessage (file:///usr/lib/code/out/vs/workbench/workbench.main.js:3233:270)
    at file:///usr/lib/code/out/vs/workbench/workbench.main.js:3232:74
    at e.fire (file:///usr/lib/code/out/vs/workbench/workbench.main.js:175:76)
    at a (file:///usr/lib/code/out/vs/workbench/workbench.main.js:311:740)
    at Socket.n._socketDataListener (file:///usr/lib/code/out/vs/workbench/workbench.main.js:311:959)
    at emitOne (events.js:116:13)
    at Socket.emit (events.js:211:7)
    at addChunk (_stream_readable.js:263:12)
    at readableAddChunk (_stream_readable.js:250:11)
    at Socket.Readable.push (_stream_readable.js:208:10)
    at Pipe.onread (net.js:594:20)
errors.ts:48 WARNING: Promise with no error callback:157
errors.ts:49 {exception: TypeError: Converting circular structure to JSON
    at JSON.stringify (<anonymous>)
    at Function…, error: null, promise: n.C…s.d…e.supportedForProcessing, handler: undefined, id: 157, …}
errors.ts:51 TypeError: Converting circular structure to JSON
    at JSON.stringify (<anonymous>)
    at Function.e.serializeReplyOK (rpcProtocol.ts:592)
    at rpcProtocol.ts:246
    at n.Class.derive._creator.then (winjs.base.js:1587)
    at e._receiveRequest (rpcProtocol.ts:241)
    at e._receiveOneMessage (rpcProtocol.ts:186)
    at rpcProtocol.ts:115
    at e.fire (event.ts:140)
    at a (ipc.net.ts:106)
    at Socket.n._socketDataListener (ipc.net.ts:135)
    at emitOne (events.js:116)
    at Socket.emit (events.js:211)
    at addChunk (_stream_readable.js:263)
    at readableAddChunk (_stream_readable.js:250)
    at Socket.Readable.push (_stream_readable.js:208)
    at Pipe.onread (net.js:594)

Screenshots

Environment

• Extension Name: vim
• Extension Version: 0.16.12
• OS Version: Linux x64 4.18.16-arch1-1-ARCH
• VSCode version: 1.27.2

Start by reproducing the crash with File: Focus on Outline, arrow-key navigation, and Enter, then inspect the reported rpcProtocol.ts serialization error. Trace which VSCodeVim request returns a circular structure and verify that Vim bindings still work after jumping to an outline element.