[Discuss-gnuradio] runtime error while running flowgraph

I created a block using gr_modtool add -l python -tsync command, where I tried to implement this discussion:[Discuss-gnuradio] Change frequency in USRP source automatically (https://lists.gnu.org/archive/html/discuss-gnuradio/2016-03/msg00402.html)

And then used this block in a flowgraph connecting to the block USRP Source.

when I tried to run the top_block.py block, the following error was displayed:-

root@pglab1-HCL-Desktop:/home/pglab1# python top_block.py

Traceback (most recent call last):

File "top_block.py", line 98, in <module>

main()

File "top_block.py", line 86, in main

tb = top_block_cls()

File "top_block.py", line 64, in __init__

self.tutorial_frew_sweep_v1_f_0 = tutorial.frew_sweep_v1_f()

File "/usr/local/lib/python2.7/dist-packages/tutorial/frew_sweep_v1_f.py", line 40, in __init__

self.set_msg_handler(pmt.intern('clock'),self.handler)

File "/usr/local/lib/python2.7/dist-packages/gnuradio/gr/gateway.py", line 200, in set_msg_handler

self.__gateway.set_msg_handler_feval(which_port, handler)

File "/usr/local/lib/python2.7/dist-packages/gnuradio/gr/runtime_swig.py", line 6423, in set_msg_handler_feval

return _runtime_swig.block_gateway_sptr_set_msg_handler_feval(self, which_port, msg_handler)

RuntimeError: attempt to set_msg_handler_feval() on bad input message port!

Can someone help me to correct this error?

Thanking you

Rensi Sam

Anna University