Hi Lucia,
thank you very much for posting the information to build fuzzylite for iOS.
Since you have created an empty project using Xcode, I guess that you did not declare the compiler directive FL_CPP11. I believe that, if you declare it, you will be able to build fuzzylite for iOS without changing the fl::null to NULL.
Would you mind checking if I am right? That is, change back the fl::null to its original value and declare the compiler directive FL_CPP11 in your fuzzylite library project and in your project using fuzzylite, and check if it works.
Again, thanks for posting your information!