Unfortunatelly it doesn't work like this.
I've tried the following:
A. Set the radio message trigger (Radio Alpha) and 0
setRadioMsg "null" in the init.
B. Set the radio message trigger (Radio Alpha) and 0
setRadioMsg "null" in another trigger activated by anybody.
C. Set the radio message trigger (Radio Alpha) and 0
setRadioMsg "null" in a script activated by an anybody trigger 8 sec after mission start.
Maybe I'm doing something wrong but it doesn't want to work!