Steps to reproduce:

1. Have item with pre-processing step "Discard unchanged" or "Discard unchanged with heartbeat". (Can use version to easier reproduce error).
2. Have value mapping added
3. Open "Test" form
4. Run test.
5. Run test again (there is Previous value present) and new value matches previous one.

Result:
Request fails with exception. There is no result. Exception: 

Uncaught TypeError: CValueMapHelper::applyValueMap(): Argument #2 ($value) must be of type string, null given, called in /var/www/html/app/controllers/CControllerPopupItemTestSend.php on line 475 and defined in /var/www/html/include/classes/helpers/CValueMapHelper.php:31 Stack trace: #0 /var/www/html/app/controllers/CControllerPopupItemTestSend.php(475) 

Expected:
Result should be "No value". Need to decide what to do with value mapping - I assume value mapping should be fixed and not displayed.