For me, this happens because my API request in Postman is being intercepted by a debugger breakpoint in my app... leaving the request hanging. If I cancel the request in Postman before killing my app's server, the error does not happen in the first place.
--> So try cancelling any open requests you are making in other programs.
On macOS, I have been using sudo lsof -t -i tcp:8000 | xargs kill -9
when I forget to cancel the open http request in order to solve error = That port is already in use.
This also, complete closes my Postman app, which is why my first solution is better.