The crash is because you arent giving "file_str_insert" a string.
When you trying to call it like this
file_str_insert(RW_file,"A",1,1);
then you are only giving it a single byte of type "CHAR" when it needs a "STRING" type.

To achieve this simply, callthe function like so
file_str_insert(RW_file, _str("A") ,1,1);
will give the function a temporary string, which is good enough...