On the Comments tab add/edit functionality, clicking Save, Upload & delete, Preview, and Cancel with "No contents/empty" text area gives an error message.

This issue can be replicated reliably using (3.6dev+ (Build: 20181031). Please see steps below:
1. Create a course.
2. Turn editing on.
3. Add a Wiki activity and create your first page and save (creole or nwiki format).
4. Go to the Wiki activity, and click the "Comments" tab.
5. Click the "Add comment" link.
6. Do not type anything in the text-area/leave it blank.
7. Click "Upload & Delete".
Result: You will see the "Error writing to database". Stacktrace in attached screenshot.
8. Repeat steps 5 and 6. Click "Preview".
Result: You will see the "Error writing to database". Stacktrace in attached screenshot.
9. Repeat steps 5 and 6. Click "Cancel".
Result: You will see the "Error writing to database". Stacktrace in attached screenshot.
10. Repeat steps 5 and 6. Click "Save".
Result: You will see the "Error writing to database". Stacktrace in attached screenshot.