我的main.swift中有以下代码:letstrategist=GKMinmaxStrategist()strategist.gameModel=positionstrategist.maxLookAheadDepth=1strategist.randomSource=nilletmove=strategist.bestMoveForActivePlayer()...其中position是我的GKGameModel子类Position的实例。运行此代码后,move为nil。bestMoveForPlayer(position.activePlayer!)也会导致nil(但positi