Crash: Performing scale changes when nothing is selected

[BagelOrb]

Steps to reproduce

• (optional) load model
• do not select model or deselect model
• click scale to open the scale menu
• enter a new number
• hit enter
• watch the world crumble

Log:

Version: master
Platform: Linux-3.19.0-26-generic-x86_64-with-Ubuntu-15.04-vivid
Qt: 5.4.1
PyQt: 5.4.1

Exception:
Traceback (most recent call last):
File "/home/tim/Development/Uranium/UM/Qt/Bindings/ActiveToolProxy.py", line 61, in setProperty
option_setter(value)
File "/home/tim/Development/Uranium/UM/../plugins/Tools/ScaleTool/ScaleTool.py", line 236, in setScaleX
obj = Selection.getSelectedObject(0)
File "/home/tim/Development/Uranium/UM/Scene/Selection.py", line 35, in getSelectedObject
return cls.__selection[index]
IndexError: list index out of range