PM: Error 500 after “Save” a private message as draft, if the field “Subject” >255 symbols

Steps:
1. Log in to the forum.
2. Create private message: Add 256 symbols or more to the Subject filed.(The values of other fields don't matter, can be empty).
3. Click “Save”

Actual result: Page Error 500 appears
Expected result: Error message with info: the max length of the field “Subject” = 255 symbols .