uni.downloadFile 下载文件时断网，控制台报错： api.onDownloadTaskStateChange

13:48:25.935 [Vue warn]: Error in event handler for "api.onDownloadTaskStateChange": "TypeError: Cannot read property '_callbackId' of undefined"  

(found <Root>)  
13:48:25.935 TypeError: Cannot read property '_callbackId' of undefined