code: def _createLocalRouter(siteStore):
      
      ls = siteStore.findUnique(LoginSystem, default=None)
      if ls is None:
          try:
              raise UnsatisfiedRequirement()
          except UnsatisfiedRequirement:
              log.err(Failure(),
                      "You have opened a substore from a site store with no "
                      "LoginSystem.  Message routing is disabled.")
          return _NullRouter()
      return LocalMessageRouter(ls)
docs: Create an L{IMessageRouter} provider for the default case, where no
      L{IMessageRouter} powerup is installed on the top-level store.
  
      It wraps a L{LocalMessageRouter} around the L{LoginSystem} installed on the
      given site store.
  
      If no L{LoginSystem} is present, this returns a null router which will
      simply log an error but not deliver the message anywhere, until this
      configuration error can be corrected.
  
      @rtype: L{IMessageRouter}