Add additional error flag constants

src/fcore_constants.f90

@@ -29,4 +29,6 @@ module fcore_constants
  integer(int32), parameter :: FCORE_EXISTING_KEY_ERROR = 10008
  !> @brief Occurs if a data type error is encountered.
  integer(int32), parameter :: FCORE_DATA_TYPE_ERROR = 10009
+ !> @brief Occurs if an attempt to write to a null reference is made.
+ integer(int32), parameter :: FCORE_NULL_REFERENCE_ERROR = 10010
 end module