Exception clicking reload cellrender of category name in plugins preferences

r4090

When i click on the reload cellrender of the category name in plugins preferences, i get this error:

Traceback (most recent call last):
  File "/home/***/exaile/xlgui/preferences/plugin.py", line 170, in on_reload_cellrenderer_clicked
    self.plugins.load_plugin(plugin, reload=True)
  File "/home/***/exaile/xl/plugins.py", line 80, in load_plugin
    path = self.__findplugin(pluginname)
  File "/home/***/exaile/xl/plugins.py", line 71, in __findplugin
    path = os.path.join(dir, pluginname)
  File "/usr/lib64/python2.6/posixpath.py", line 65, in join
    if b.startswith('/'):
AttributeError: 'NoneType' object has no attribute 'startswith'

Beyond the error maybe that cellrender is useless for the category name row, or do you think could be useful to implement the ability to reload all the plugins belonging to a certain category?