  function withdrawTradingFeeOwner() public onlyOwner returns (string memory){
      uint256 amount = availableTradingFeeOwner();
      require (amount > 0, 'Nothing to withdraw');
      tokens[address(0)][feeAccount] = 0;
      msg.sender.transfer(amount);
      emit OwnerWithdrawTradingFee(owner, amount);
  }