Currently as a user I cannot send emails with the template configuration which includes "simple" render type and a template file.

Steps to reproduce:

1. Configure SMTP server via Mail Tools app
2. Go to /modules/mail/config/templateConfiguration and then create a template configuration with type = simple and a specific templateFile = xxx (Refer to the Template-configuration.png attachment below)
3. Test the template using Mail Tools app.
(Go to Mail Tools -> Select Verify Setup tab -> Choose the template -> Click "SEND TEST EMAIL" button)

Expected result: The email based on the configured template file is sent successfully.
Actual result: The email is not sent, the error message "Unable to send email: null." is displayed. (Refer to the Error.png attachment below)

For Mail configuration and testing refer to: Mail module