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Login as admin
Create a course
Enrol yourself as a student
From the participants list, select your account
Select With selected users... > Send message
Without entering any message content, press Send message to 1 person
Confirm you see error "Please enter message text"
Enter some message text
Press Send message to 1 person
Confirm you see the message "Message sent to 1 person"
Open messaging drawer
Confirm you received the message
Check your browser console, confirm there are no exceptions regarding "Notification is not defined"