Replace Null values with continuous numbering

[GloriousWater]

Hello. I would love some inputs on how to solve the following.

I have attached some data to illustrate before, after and desired result. Its a list of different courses. This list, is updated by joining the current list, with the "new" unique course names from a different list. However when adding the new course names, they do not have a course ID so they are given the value Null. 

Problem: Replace null with the previous max course ID number + 1. 

I have only been able to replace the null values with the same value. e.g. both the new courses would be ID 14.

Hope you can help. 

*edit typo. +clarification. 

Best Regards Simon. 

[JosephSerpis · Accepted solution]

Hi @GloriousWater I mocked up a workflow let me know what you think?

[GloriousWater]

Amazing. that solves it. Thanks a lot. Saved me so much frustration. I had been looking a bit at the multi tools but couldn't quite figure them.