Sorting instances before using name popover causes index error

[ryanbugden]

There seems to be an index mismatch after sorting, when dealing with the new popover from #92

Reproduce:

• Sort Instances by something like weight
• If the sort order has changed from what what loaded initially, double-click the [i] icon to get into the pop-over on all instances.

One of the interactions will cause this traceback.

Current workaround:

• Close DSE2 and reopen, don't sort.

Traceback (most recent call last):
  File "lib/doodleDelegate.pyc", line 90, in sendEvent_
  File "lib/cells/doubleClickCell.pyc", line 21, in hitTestForEvent_inRect_ofView_
  File "/Users/Ryan/Library/Application Support/RoboFont/plugins/DesignspaceEditor2.roboFontExt/lib/designspaceEditor/ui.py", line 1463, in instancesListDoubleClickCallback
    self.instancesPopover = InstancesAttributesPopover(self.instances.list, self.operator, closeCallback=self.instancesChanged)
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/Ryan/Library/Application Support/RoboFont/plugins/DesignspaceEditor2.roboFontExt/lib/designspaceEditor/ui.py", line 435, in __init__
    index = listView.getSelection()[0]
            ~~~~~~~~~~~~~~~~~~~~~~~^^^
IndexError: list index out of range