Issue with offscreen CodeMirror

[curran]

We're getting this error:

index-DwusECPl.js:119 Error finding element position: RangeError: Trying to find position for a DOM position outside of the document
    at CL.posFromDOM (index-DwusECPl.js:62:49477)
    at St.posAtDOM (index-DwusECPl.js:65:14285)
    at HTMLDocument.b (index-DwusECPl.js:119:19686)

I think it might be from a certain presence indicator plugin that is trying to render something on an off-DOM instance of CodeMirror. Perhaps we can just wrap all the code that uses posFromDOM or posAtDOM in a try/catch?