Code is wrong, performing some invalid assignment. (You have to fix the code.)
Code is correct, but your application is not compiled correctly. (You should compile forward FormLetter class, or - because you may have such issues in more places - compile the whole AOT.)
You need not to compile the whole application, rather compile(use forward compile for compiling the classes that are down in the inheritance hierarchy) the problematic classes.
You can do the parallel compile(if you are using R2 CU7 and versions after that).