List的touchItem报错问题

list滑动的时候好像会触发touchItem， 代码中

public GObject touchItem
{
get
{
//find out which item is under finger
//逐层往上知道查到点击了那个item
GObject obj = GRoot.inst.touchTarget;
GObject p = obj.parent;
while (p != null)
{
if (p == this)
return obj;

obj = p;
p = p.parent;
}

return null;
}
}

GObject p = obj.parent 会报空引用异常，应该是无obj，不清楚具体原因。。求谷主解答。。Unity + tolua#