So, the good folks at CODESYS help sent the following in response to my ticket:Your problem is that 'NULL' is declared in two different libraries (CDS_MemMan, SoftMotion_CNC_Constants) , both of which do not use the 'qualified_only' attribute. Therefore, there are ambiguous messages for NULL and the related-position messages to track down the problem. In your case, you have to qualify the 'NULL' you want to use.
A bit cryptic, but the answer to the problem is in there. I was able to find the CDS_MemMan library definition of NULL, but i wasn't able to find SoftMotion_CNC_Constants exactly. However, I did find another definition for NULL in the SM3_Basic library. The only way i have been able to 'qualify' anything in the library properties was the 'Only allow qualified access to all identifiers' checkbox (see pic). One or the other library needs to be checked off.
This has broken the log jam. The project now compiles without errors, downloads to the simulation, and runs. There is, however, still some ambiguity going on somewhere because, in my case anyway, every NULL is still underlined and pops up an "Ambiguous use of name 'NULL'" message. That doesn't stop compilation, and for now i'll take that as a win.
Find the new revision of the Fuzzy RPi project (Fuzzy_eFFL_RPi_010.project) at: https://1drv.ms/f/s!AgeXD5P3kwJIg3Y4BOsecXPf920K
Thanks to the CODESYS folks for the direction.