Ubuntu software center is not opening, when I type:

software-center

in the command line, the following errors are generated:

(software-center:8570): Gtk-WARNING **: Theme parsing error: gtk.css:227:31: Failed to import: Error opening file: No such file or directory
2012-09-30 17:00:58,068 - softwarecenter.ui.gtk3.app - INFO - setting up proxy 'http://10.3.100.212:8080/'
2012-09-30 17:00:58,071 - softwarecenter.db.database - INFO - open() database: path=None use_axi=True use_agent=True
2012-09-30 17:00:58,327 - softwarecenter.backend.reviews - WARNING - Could not get usefulness from server, no username in config file
2012-09-30 17:00:58,428 - softwarecenter.ui.gtk3.app - INFO - show_available_packages: search_text is '', app is None.
2012-09-30 17:00:58,433 - softwarecenter.db.pkginfo_impl.aptcache - INFO - aptcache.open()
Traceback (most recent call last):
  File "/usr/share/software-center/softwarecenter/db/pkginfo_impl/aptcache.py", line 243, in open
    self._cache = apt.Cache(GtkMainIterationProgress())
  File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 102, in __init__
    self.open(progress)
  File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 145, in open
    self._cache = apt_pkg.Cache(progress)
SystemError: E:Encountered a section with no Package: header, E:Problem with MergeList /var/lib/apt/lists/ppa.launchpad.net_webupd8team_themes_ubuntu_dists_precise_main_binary-i386_Packages, E:The package lists or status file could not be parsed or opened.
2012-09-30 17:01:00,130 - softwarecenter.db.enquire - ERROR - _get_estimate_nr_apps_and_nr_pkgs failed
Traceback (most recent call last):
  File "/usr/share/software-center/softwarecenter/db/enquire.py", line 115, in _get_estimate_nr_apps_and_nr_pkgs
    tmp_matches = enquire.get_mset(0, len(self.db), None, xfilter)
  File "/usr/share/software-center/softwarecenter/db/appfilter.py", line 89, in __call__
    if (not pkgname in self.cache and
  File "/usr/share/software-center/softwarecenter/db/pkginfo_impl/aptcache.py", line 263, in __contains__
    return self._cache.__contains__(k)
AttributeError: 'NoneType' object has no attribute '__contains__'
Traceback (most recent call last):
  File "/usr/bin/software-center", line 176, in <module>
    app.run(args)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 1422, in run
    self.show_available_packages(args)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 1352, in show_available_packages
    self.view_manager.set_active_view(ViewPages.AVAILABLE)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/session/viewmanager.py", line 154, in set_active_view
    view_widget.init_view()
  File "/usr/share/software-center/softwarecenter/ui/gtk3/panes/availablepane.py", line 171, in init_view
    self.apps_filter)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/catview_gtk.py", line 238, in __init__
    self.build(desktopdir)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/catview_gtk.py", line 511, in build
    self._build_homepage_view()
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/catview_gtk.py", line 271, in _build_homepage_view
    self._append_whats_new()
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/catview_gtk.py", line 450, in _append_whats_new
    whats_new_cat = self._update_whats_new_content()
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/catview_gtk.py", line 439, in _update_whats_new_content
    docs = whats_new_cat.get_documents(self.db)
  File "/usr/share/software-center/softwarecenter/db/categories.py", line 124, in get_documents
    nonblocking_load=False)
  File "/usr/share/software-center/softwarecenter/db/enquire.py", line 317, in set_query
    self._blocking_perform_search()
  File "/usr/share/software-center/softwarecenter/db/enquire.py", line 212, in _blocking_perform_search
    matches = enquire.get_mset(0, self.limit, None, xfilter)
  File "/usr/share/software-center/softwarecenter/db/appfilter.py", line 89, in __call__
    if (not pkgname in self.cache and
  File "/usr/share/software-center/softwarecenter/db/pkginfo_impl/aptcache.py", line 263, in __contains__
    return self._cache.__contains__(k)
AttributeError: 'NoneType' object has no attribute '__contains__'