Non-existing CallableTool does not bring down the portal

During erp_core upgrade, my instance failed on non-existing document.erp5.CallableTool. So foolish me created Portal Component "document.erp5.CallableTool" which content was rather simple

class CallableTool:
  pass

That gave me infinite loop in acquisition chain and brought my whole portal down. The correct implementation would be

from product.ERP5Type.Tool.BaseTool import BaseTool
class CallableTool(BaseTool):
  pass

@klaus and @tatuya please review if this fix is necessary or even good because I am unsure why it actually fixed my problem.