fix issue #540: the problem was located in the stack handling routine. After...

fix issue #540: the problem was located in the stack handling routine. After the fist event in the stack was saved (in events file), (#541)

the routine checks if the record has to be published in the NC. This check generates an error taht made the routine to stop the
handling of the other events. The generated error was due to a NULL (0) value returned as a std::string.