exited with reason undef

[akshay-ans]

Lager event handler {lager_file_backend,"test.log"} exited with reason {'EXIT',{undef,[{rfc3339,format,[1547570271057879,micro_seconds],[]},{lager2json,timestamp,1,[{file,"/var/log/rabbitmq/log/lager2json/src/lager2json.erl"},{line,40}]},{lager2json,msg_to_map,1,[{file,"/var/log/rabbitmq/log/lager2json/src/lager2json.erl"},{line,28}]},{lager2json,format,2,[{file,"/var/log/rabbitmq/log/lager2json/src/lager2json.erl"},{line,12}]},{lager_file_backend,handle_event,2,[{file,"src/lager_file_backend.erl"},{line,181}]},{gen_event,server_update,4,[{file,"gen_event.erl"},...]},...]}}