苹果iOS13 内购隐藏大坑：
- (void)productsRequest:(SKProductsRequest *)request didReceiveResponse:(SKProductsResponse *)response
iOS12之前是在主线程回调，
在iOS13改成了在一个异步线程回调，如果在iOS13下苹果内购这个回调直接执行一些UI操作，则必然会提示
Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread.