Download fails immediately

If I start a download to download an apk, if fails directly.
```
W/WM-WorkSpec( 9130): Backoff delay duration less than minimum value
E/WM-WorkerWrapper( 9130): Work [ id=0d0d1fc7-9ae9-4511-b080-0c8e0c236594, tags={ vn.hunghd.flutterdownloader.DownloadWorker, flutter_download_task } ] failed because it threw an exception/error
E/WM-WorkerWrapper( 9130): java.util.concurrent.ExecutionException: java.lang.NoSuchMethodError: No virtual method getContentLengthLong()J in class Ljava/net/HttpURLConnection; or its super classes (declaration of 'java.net.HttpURLConnection' appears in /system/framework/core-libart.jar)
E/WM-WorkerWrapper( 9130): at androidx.work.impl.utils.futures.AbstractFuture.getDoneValue(AbstractFuture.java:516)
E/WM-WorkerWrapper( 9130): at androidx.work.impl.utils.futures.AbstractFuture.get(AbstractFuture.java:475)
E/WM-WorkerWrapper( 9130): at androidx.work.impl.WorkerWrapper$2.run(WorkerWrapper.java:298)
E/WM-WorkerWrapper( 9130): at androidx.work.impl.utils.SerialExecutor$Task.run(SerialExecutor.java:91)
E/WM-WorkerWrapper( 9130): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
E/WM-WorkerWrapper( 9130): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
E/WM-WorkerWrapper( 9130): at java.lang.Thread.run(Thread.java:818)
E/WM-WorkerWrapper( 9130): Caused by: java.lang.NoSuchMethodError: No virtual method getContentLengthLong()J in class Ljava/net/HttpURLConnection; or its super classes (declaration of 'java.net.HttpURLConnection' appears in /system/framework/core-libart.jar)
E/WM-WorkerWrapper( 9130): at vn.hunghd.flutterdownloader.DownloadWorker.downloadFile(DownloadWorker.java:309)
E/WM-WorkerWrapper( 9130): at vn.hunghd.flutterdownloader.DownloadWorker.doWork(DownloadWorker.java:201)
E/WM-WorkerWrapper( 9130): at androidx.work.Worker$1.run(Worker.java:85)
E/WM-WorkerWrapper( 9130): ... 3 more
I/WM-WorkerWrapper( 9130): Worker result FAILURE for Work [ id=0d0d1fc7-9ae9-4511-b080-0c8e0c236594, tags={ vn.hunghd.flutterdownloader.DownloadWorker, flutter_download_task } ]
```
It only appears in version `1.6.0-nullsafety.0`, before everything was working. I think this [change](https://github.com/fluttercommunity/flutter_downloader/commit/3d8be89d91028e615c8b68d3ff022fec461c4a4a#diff-c2611680d5e87fd1a5008e056d7190f2198b3962445557296c76dd2d650c9cacR301) is responsible.

Flutter doctor:
```
[✓] Flutter (Channel beta, 2.1.0-12.2.pre, on Linux, locale de_DE.UTF-8)
• Flutter version 2.1.0-12.2.pre at /home/vinzent/snap/flutter/common/flutter
• Framework revision 5bedb7b1d5 (vor 9 Tagen), 2021-03-17 17:06:30 -0700
• Engine revision 711ab3fda0
• Dart version 2.13.0 (build 2.13.0-116.0.dev)

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
• Android SDK at /home/vinzent/Android/Sdk
• Platform android-30, build-tools 30.0.3
• Java binary at: /usr/local/android-studio/jre/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
• All Android licenses accepted.
```