Adding SSH site in Jenkins throws exception
It seems to be a known, but not yet fixed bug JENKINS-46172
I've experienced this error when because of errors on page had left unspecified ssh credentials.
In my case "Credentials" list was empty and "Add" button didn't work.
So I had to go to "Jenkins/Credentials" menu and enter credentials there.It made possible to select credentials from list and ssh host addition went without described error.