Jenkins with Docker plugin java.lang.NullPointerException: uri was not specified
Had the same issue, for me it was a stupid little thing.
Enter the URI: http://127.0.0.1:4243 or 2375 for the new version and click Apply first before you Test the connection.
Current plugin ver 1.39 doesn't accept http url format. Only tcp://127.0.0.1:2375 works fine.