PM: Error 500 appears after trying to save a private message as draft if "To" contains not registered username

Steps to reproduce

1. Go to private messages
2. Click "Compose"
3. Fill field "To" with login that not registered
4. Fill field "Subject" with some valid symbols
5. Fill field "Message content" with some valid symbols
6. Click "Save"

Actual result: Page Error 500 appears
Expected result: Message has been saved