chapter5. Router :: how to refresh the same page?

[tudoistube]

Hi, long time again to visit for searching a help.

In an example of chapter 5/routing, the 1st click to any repository is working well.

But, when I refresh the repository, I have a problem with a single message
"Cannot GET /repo/pro-react.github.io" on browser as below attached images.

before refresh >

after refresh >

I compared my source with your example source but found no difference.
(my practised source of App.js is here : https://github.com/tudoistube/zbook_proreact/blob/by_203p/app/App.js)

Would you tell me what to fix for refreshing?

Thanks for your reading.