(closes#5295)
The cause of this crash was:
- when cancelling a line, `drawWay.cancel()` needs to clean up old edits.
- it does this by looping `context.pop()` them off history until they are gone
- each `context.pop()` dispatches a 'change' event
- if we reset the chapter on one of these change events, it throws away the
graph, and `drawWay.cancel()` can't finish what it's doing
a6b3265189