The function fillOrder set order status as filled after external token transfer.
https://docs.openzeppelin.com/contracts/3.x/erc777
This opens room for reentrancy if the token is ERC777 token when external user can control the execution flow via the receive hook.
The external token transfer is this line of code:
Where order.recipient can re-enter the fillOrder to fill order more than one times it is recommended to add nonReentrant modifer to the function fillOrder.
