Fail to accept URL as XMLTV source

The problem seem to be in XMLTVSource.__init__() from script.tvguide/source.py, which have these two lines:
```
        if not self.xmltvFile or not xbmcvfs.exists(self.xmltvFile):
            raise SourceNotConfiguredException()                            
```
If only the URL is provided, the add-on refuse to start.  If I comment out those lines, the add-on start as it should.