Crash on priority criteria addition

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In the prioritize dialogs, when clicking the add button when there's nothing selected, there's a crash:

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Application Identifier: com.hardcoded-software.dupeguru
Application Version: 3.2.1

Traceback (most recent call last):
File "inter/prioritize_dialog.pyc", line 25, in addSelected
File "core/gui/prioritize_dialog.pyc", line 64, in add_selected
TypeError: list indices must be integers, not NoneType
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[ghost]

(from [5b964741a772]) [#178 state:fixed] Fixed a crash on adding a criterion with an empty selection.
https://bitbucket.org/hsoft/dupeguru/changeset/5b964741a772/