This error message appears in Magento 2 when you deleted a theme directory and the theme registry reminds in the database.

To fix this issue you need to go to app/design/frontend and check which themes you have installed.
Then go to the database and execute the following command:

SELECT * FROM theme;

Check what is the extra theme in that table and remove the registry.

Clear cache and that should be it.

Here you can find the changelog for Contact Button for WhatsApp Plugin.