Unable to connect to python OPC UA server

I have set up a OPC server using the python minimal server for the OPC UA on github. This is the message show when the correct address is entered.
“object reference not set to an instance”.
Please assist me on resolving this issue


Try to set an IP address rather than as the server endpoint, such as Next, copy and paste (or type) the server endpoint into the OPC Server field in Factory I/O. Press Return to connect. Please let me know if this works for you.