Expression error

Instead of using 

if[FY18]= #date(null)

Use

if[FY18]= null

To explain further, your error message is because:

#date(year, month, day) requires three arguments.

#date(null) has only one argument

Of course, #date(null,null,null) would also be invalid as the arguments for #date(...) need to be of type number and each within a certain range of numbers, and null cannot be converted to a number. 

As others have pointed out, you can just use if [FY18]=null ...