To Reproduce:
1. Download and open the project: TestNullable_U3.zip
2. This line fails:

int? a = (int?)ManageFriendInviteAction.Action_1; // Does not work

Creating an object of type ManageFriendInviteAction and casting works fine:

int a = ManageFriendInviteAction.Action_1;
int? b = (int?)a; // Works

The error string is:

Internal compiler error. See the console log for more information. output was:
Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object