Replacing live video background with nothing live causes crash

[Jonathan Corwin]

From: http://www.openlp.org/en/forums/openlp_20/bug_reports/error_when_setting_video_as_a_bg.html

If you right click on a video in the media manager and select Replace Live Background, whilst there is currently no live song, then the following occurs:

Traceback (most recent call last):
  File "C:\Documents and Settings\Jonathan\My Documents\bzr\openlp\trunk\build\pyi.win32\OpenLP\outPYZ1.pyz/openlp.plugins.media.lib.mediaitem", line 118, in onReplaceClick
  File "C:\Documents and Settings\Jonathan\My Documents\bzr\openlp\trunk\build\pyi.win32\OpenLP\outPYZ1.pyz/openlp.core.ui.maindisplay", line 292, in video
  File "C:\Documents and Settings\Jonathan\My Documents\bzr\openlp\trunk\build\pyi.win32\OpenLP\outPYZ1.pyz/openlp.core.ui.maindisplay", line 309, in preview
AttributeError: 'MainDisplay' object has no attribute 'serviceItem'

The Replace Live Background option should be disabled if there are currently no text-based items on the live display.