Issue title: Crash during logfile re-play
 Issue body: QGS crashes when stopping, rewinding a logfile re-play and on the end of the file.

 Comments: 
 Comment 0: flyingk, can you provide more info here? I am not able to reproduce this on mac OSX using the latest QGC2 build from here:
http://www.inf.ethz.ch/personal/lomeier/downloads/qgroundcontrol-mac-64-v2.0.0-beta-build01.dmg

 Comment 1: Hi Tom,

with the latest build it seems to work now on the mac. Must have been fixed recently.
Cheers!
Kai

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