How to reproduce issue

Send a link of an article from front end on a clean install of Joomla! 3.9 (Click on the send email icon at the top right of an article. A pop up appears and at the top I see an error message: "Captcha plugin not set or not found. Please contact a site administrator.")

Expected result:

I can send an email with a link to an article without a captcha

Actual result

Joomla requires a captcha and gives this Error message:

Error
Captcha plugin not set or not found. Please contact a site administrator.

System information (as much as possible)

Joomla! Version
Joomla! 3.9.0-alpha Alpha [ Amani ] 31-August-2018 12:30 GMT
Joomla! Platform Version
Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
User Agent
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:62.0) Gecko/20100101 Firefox/62.0
PHP: 7.2

Additional comments