Closed file left half-open if next working set entry no longer exists

[peterflynn]

1. Open three files in the working set: A, B, and C
2. Make sure file C is selected
3. Close Brackets
4. Delete file B
5. Reopen Brackets
6. Close file C

Result: after dismissing the error dialog about being unable to open file B, you're left with nothing selected in the left-hand pane, and file C still open in an editor but no longer listed in the working set.

Although it looks pretty broken/sketchy, I think the underlying Document stuff is still in an ok state -- if you type in the "orphaned" editor, file C is re-added to the working set and saving changes works.

[peterflynn]

I think variations of this bug were always possible in rare edge cases, but pull #1197 made it much easier to hit (via the above steps). It looks ugly enough that I think we should consider fixing for Sprint 14...

[redmunds]

FBNC back to @peterflynn

[peterflynn]

Confirmed fixed