To reproduce:
1. Open any project
2. In Project window, create Assembly Definition file, select it
3. In inspector window, Version Defines section, press + to add new defines
4. Select any Resource from drop-down and click Apply
5. Observe the Version Defines section and console

Expected: selected define is added
Actual: Version Defines section is empty, console infinitely spammed with 'ArgumentNullException: Argument cannot be null" errors (until Assembly file is removed)

Reproduced in: 2019.1.0a11, 2019.1.0b4, 2019.2.0a6

Note1: in addition, Applying without having any define selected will result in console error "AssemblyDefinitionException: Invalid version define"
Note2: feature of Version Defines introduced in 2019.1.0a10