[reals] [refactor branch] subsequent candidates not shown in certain circumstances

[mtbannister]

When transitioning between successfully measured candidates on a particular file (o4h53_uh60_p39.measure3.cands.astrom), the following error was generated:
Traceback (most recent call last):
File "/Users/bannisterm/lib/python2.7/site-packages/ossos-0.4.11-py2.7.egg/ossos/gui/views/validation.py", line 355, in _on_submit
comment)
File "/Users/bannisterm/lib/python2.7/site-packages/ossos-0.4.11-py2.7.egg/ossos/gui/controllers.py", line 428, in on_do_accept
self.view.clear()
File "/Users/bannisterm/lib/python2.7/site-packages/ossos-0.4.11-py2.7.egg/ossos/gui/views/appview.py", line 30, in new_guithread_function
return function(_args, *_kwargs)
File "/Users/bannisterm/lib/python2.7/site-packages/ossos-0.4.11-py2.7.egg/ossos/gui/views/appview.py", line 106, in clear
self.image_viewer.clear()
File "/Users/bannisterm/lib/python2.7/site-packages/ossos-0.4.11-py2.7.egg/ossos/fitsviewer/baseviewer.py", line 61, in clear
self.ds9.set("frame delete all")
File "/Users/bannisterm/lib/python2.7/site-packages/pyds9.py", line 532, in set
s, blen, 1)
File "/Library/Python/2.7/site-packages/xpa.py", line 156, in xpaset
if errmsg: raise ValueError, errmsg
ValueError: XPA$ERROR can't read "end(contrib)": variable isn't array (DS9:validate ce57b254:60120)

This is after it has successfully generated the .mpc file for that candidate.
The next candidate seems to be present (the MOP gui is showing values) so it has loaded correctly, but the ds9 shows completely white for all three frames.

It seems to be possible to smoothly continue working by making a clean exit as normal, and restarting validate on the same target folder, at which point the error did not reoccur after the second time through.