"Has there been any harm done"
Thats sure possible, if you started her up and the genny was polarized opposite from that of the battery orientation, YOU COULD HAVE DAMAGED THE VOLTAGE REGULATOR...... Theres also a chance
it may have survived such an incident CROSS YOUR FINGERS
"and how can i correct the situation"
You can correct it (if VR is still okay) by polarizing the genny BEFORE you start it.
The genny can work fine at EITHER polarity provided its properly polarized.
If the battery and ammeter are correct, if you turn on lights or ignition when shes setting there not running the ammeter should swing over to - discharge, then once running at RPM if all is working and correct it should, of course, swing to + charge