Missing position existence check leads to misleading errors

In the closePosition function, there is no validation that the positionId exists before accessing it:

function closePosition(uint positionId) external nonReentrant {
    Position storage position = positions[positionId];

    require(position.walletAddress == msg.sender, "Only position creator may modify position");
    require(position.open == true, "Position is already closed");
    // ...
}

If a user provides a positionId >= currentPositionId, the position will be empty (all fields zero-initialised). This leads to a misleading error message: if msg.sender is rightfully trying to close his position but entered the wrong positionId, they see "Only position creator may modify position".

Add an explicit existence check at the start of the function:

require(positionId < currentPositionId, "Position does not exist");