def METHOD_NAME(self, context):
    n = context.node
    named = self.obj_name
    if named == "__SV_INSTANCE_RESET__":
        n.mesh_to_clone = ""
        n.has_instance = False
    else:
        # we assume these objects have not disappeared in the mean time.
        n.mesh_to_clone = bpy.data.objects[named].data.name
        n.has_instance = True
    return {'FINISHED'}