code: func (h *Handler) isValidTxSim(channelID string, txid string, fmtStr string, args ...interface{}) (*TransactionContext, error) {
  	txContext := h.TXContexts.Get(channelID, txid)
  	if txContext == nil || txContext.TXSimulator == nil {
  		err := errors.Errorf(fmtStr, args...)
  		chaincodeLogger.Errorf("no ledger context: %s %s\n\n %+v", channelID, txid, err)
  		return nil, err
  	}
  	return txContext, nil
  }
docs: // is this a txid for which there is a valid txsim