Fix Sound Message Nodes crashing the exporter. (#368)

If a sound message was being sent to a random sound modifier, the
responder tree might crash on export. This fixes that particular problem
and adds a mitigation to the responder code itself. Remember kids that,
in Python `None and True` is `None`...

korman/nodes/node_messages.py

@@ -893,6 +893,7 @@ class PlasmaSoundMsgNode(idprops.IDPropObjectMixin, PlasmaMessageWithCallbacksNo
     def has_callbacks(self):
         if not self.is_random_sound:
             return self.event != "NONE"
+        return False
 
     @classmethod
     def _idprop_mapping(cls):

korman/nodes/node_responder.py

@@ -356,7 +356,7 @@ class PlasmaResponderStateNode(PlasmaNodeBase, bpy.types.Node):
         """
         if node is None:
             node = self
-        return sorted(node.find_outputs("msgs"), key=lambda x: x.has_callbacks and x.has_linked_callbacks)
+        return sorted(node.find_outputs("msgs"), key=lambda x: bool(x.has_callbacks and x.has_linked_callbacks))
 
 
 class PlasmaRespStateSocket(PlasmaNodeSocketBase, bpy.types.NodeSocket):