When ever I run roscore, this error prompts. How can I fix this issue ? Please provide a solution.

edit

Error:

   Traceback (most recent call last):
      File "/opt/ros/melodic/bin/roscore", line 36, in <module>
        from rosmaster.master_api import NUM_WORKERS
      File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/__init__.py", line 35, in <module>
        from .main import rosmaster_main
      File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/main.py", line 43, in <module>
        import rosmaster.master
      File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/master.py", line 47, in <module>
        import rosmaster.master_api
      File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/master_api.py", line 72, in <module>
        from rosmaster.util import xmlrpcapi
      File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/util.py", line 48, in <module>
        from defusedxml.xmlrpc import monkey_patch
    ModuleNotFoundError: No module named 'defusedxml'

I'm using Ubuntu 18.04. and tried installing defusedxml. but no use

Thanks in advance