Hi Anil,
The exception is because you have created outbound plug op_find_contact in the view BP_DATA/AccountContactsOV and i can see from the code that you are trying to call the outbound plug of the window as below,
lr_window->call_outbound_plug( iv_outbound_plug = 'FIND_CONTACT').
It is trying to find the outbound plug in the window with name 'FIND_CONTACT'(which is not available in window)
But the outbound plug crated (in step3) in the window BP_DATA/Contacts is op_outbound which is different from the view plug name.
This outbound plug needs to be changed to 'FIND_CONTACT'.
So you need to create an outbound plug in the window BP_DATA/Contacts must be similar to the outbound plug of the view.
Thanks,
Vijay