Open LP 2.4.6 - crash on Extract Song Usage Data from startup

[Richard Smith]

**OpenLP Bug Report**
Version: {'version': '2.4.6', 'full': '2.4.6', 'build': None}

--- Details of the Exception. ---

Having upgraded OpenLP from 2.4.5 to 2.4.6, this bug is showing again. When I selected Extract Usage Data, I was not presented with a dialog to set dates, it just went to this error.

 --- Exception Traceback ---
Traceback (most recent call last):
  File "/Applications/OpenLP.app/Contents/MacOS/plugins/songusage/songusageplugin.py", line 226, in on_song_usage_report
    self.song_usage_detail_form.initialise()
  File "openlp/plugins/songusage/forms/songusagedetailform.py", line 58, in initialise
TypeError: setSelectedDate(self, Union[QDate, datetime.date]): argument 1 has unexpected type 'str'

--- System information ---
Platform: Darwin-16.5.0-x86_64-i386-64bit

--- Library Versions ---
Python: 3.4.6
Qt5: 5.6.2
PyQt5: 5.7.1
QtWebkit: 538.1
SQLAlchemy: 1.1.5
SQLAlchemy Migrate: -
BeautifulSoup: 4.4.1
lxml: 3.6.0
Chardet: 2.3.0
PyEnchant: 1.6.6
Mako: 1.0.6
pyICU: -
pyUNO bridge: -
VLC: -

[Phill (phill-ridout)]

see also: https://bugs.launchpad.net/openlp/+bug/1661416