rqt AttributeError: 'NoneType' object has no attribute 'strip'

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Hello! I am trying to run rqt_gragh as the tutorials/understanding topics said. But when I run that I got following error.

   human@pangaea:~$ rosrun rqt_gui rqt_gui 
Traceback (most recent call last):
  File "/opt/ros/groovy/lib/rqt_gui/rqt_gui", line 8, in <module>
    p = r.get_path('rqt_gui')
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 188, in get_path
    self._update_location_cache()
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 171, in _update_location_cache
    list_by_path(self._manifest_name, path, cache)
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 68, in list_by_path
    resource_name = root.findtext('name').strip(' \n\r\t')
AttributeError: 'NoneType' object has no attribute 'strip'

I would really appreciate if anybody can tell me how to figure it out.Thanks!