A withdraw function for Token Sale

I was trying my hand on the Capture the Ether Levels. And was thinking there should be a withdrawal function that allows withdrawal of excess balance, as the remaining ether, after the level is finished, is locked in that contract forever. I know it is testnet eth, but still.

A simple implementation could be like the below:

function withdraw() public {
    require(isComplete(), "The level is still not solved");
    msg.sender.transfer(address(this).balance);
}

Anyone can call the above function, so that might be a potential vulnerability ;) But still better than those tokens being locked.