fix safeguard functions

lib/downloadTask.js

@@ -57,24 +57,24 @@ export default class DownloadTask {
 
   onBegin ({ expectedBytes, headers }) {
     this.state = 'DOWNLOADING'
-    this?.beginHandler({ expectedBytes, headers })
+    this.beginHandler?.({ expectedBytes, headers })
   }
 
   onProgress (percent, bytesWritten, totalBytes) {
     this.percent = percent
     this.bytesWritten = bytesWritten
     this.totalBytes = totalBytes
-    this?.progressHandler(percent, bytesWritten, totalBytes)
+    this.progressHandler?.(percent, bytesWritten, totalBytes)
   }
 
   onDone ({ location }) {
     this.state = 'DONE'
-    this?.doneHandler({ location })
+    this.doneHandler?.({ location })
   }
 
   onError (error, errorCode) {
     this.state = 'FAILED'
-    this?.errorHandler(error, errorCode)
+    this.errorHandler?.(error, errorCode)
   }
 
   pause () {