Fix typo in default process_execute

wrappers/modelverse.py

@@ -436,7 +436,7 @@ def all_instances(model_name, type_name, context=None):
     INPUT("all_instances", context, [model_name, type_name])
     return OUTPUT()
 
-def process_execute(process_name, model_mapping, callbacks=None):
+def process_execute(process_name, model_mapping, callbacks={}):
     # for all callbacks to SCs, start up the output port already
     sc_ports = {}
     for k, v in callbacks.items():