FATAL EXCEPTION DownloadWorker.java:94

Hi,

Exception is causing the application to close,
detail: the internet is slow
```
E/AndroidRuntime(30589): FATAL EXCEPTION: pool-1-thread-1
E/AndroidRuntime(30589): Process: com.upaki.medmidia, PID: 30589
E/AndroidRuntime(30589): java.lang.NullPointerException: Attempt to read from field 'int vn.hunghd.flutterdownloader.DownloadTask.primaryId' on a null object reference
E/AndroidRuntime(30589): at vn.hunghd.flutterdownloader.DownloadWorker.doWork(DownloadWorker.java:94)
E/AndroidRuntime(30589): at androidx.work.Worker$1.run(Worker.java:84)
E/AndroidRuntime(30589): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1162)
E/AndroidRuntime(30589): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:636)
E/AndroidRuntime(30589): at java.lang.Thread.run(Thread.java:764)
I/Process (30589): Sending signal. PID: 30589 SIG: 9
Lost connection to device.
```

Flutter version:
```
Flutter 1.6.3 • channel beta • https://github.com/flutter/flutter.git
Framework • revision bc7bc94083 (6 weeks ago) • 2019-05-23 10:29:07 -0700
Engine • revision 8dc3a4cde2
Tools • Dart 2.3.2 (build 2.3.2-dev.0.0 e3edfd36b2)
```

Downloader version:
```
flutter_downloader: 1.1.4
```

Start with DownloadWorker.java at line 94 and inspect how the worker behaves when the internet is slow and its DownloadTask lookup returns null. Reproduce the failure using the reported Flutter 1.6.3 and flutter_downloader 1.1.4 versions, then verify that a slow connection no longer closes the application.

Written by the indexing model from the issue text.