Exception when creating overlay

[csatt]

This is an obscure case.

In Wizard:
- Select a run
- "Change Title" -> "Foo"
- "Change Title" -> "None"
- Select that run and another
- "Overlay"

Unexpected error: <type 'exceptions.AttributeError'>
Traceback (most recent call last):
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-tk/Tkinter.py", line 1536, in call
return self.func(*args)
File "./IV_Swinger2_gui.py", line 3565, in overlay_runs
self.plot_overlay_and_display()
File "./IV_Swinger2_gui.py", line 4109, in plot_overlay_and_display
self.plot_overlay()
File "./IV_Swinger2_gui.py", line 4075, in plot_overlay
self.get_overlay_curve_names()
File "./IV_Swinger2_gui.py", line 4121, in get_overlay_curve_names
name = self.master.props.overlay_names[dts].decode("utf-8")
AttributeError: 'NoneType' object has no attribute 'decode'