Hi Peter,
I suspect, your code to get the message manager reference into lo_message_manager object, is put inside some condition but in this case, that condition is not met and hence LO_MESSAGE_MANAGER object reference is NULL.
If so, please put the code to get message manager reference outside the condition.
Also, before calling any method of an object, its better to check if its filled or not
if lo_message_manager is bound.
" Your code goes here
endif.
Hope this helps you.
Regards,
Rama