这是我的代码
Calendar startCalendar;
Calendar stopCalendar;
startCalendar= Calendar.getInstance();
stopCalendar=Calendar.getInstance();
startCalendar.set(2020,10,2,0,0);
stopCalendar.set(2020,10,3,0,0);
NetworkStatsManager networkStatsManager = (NetworkStatsManager) getContext().getSystemService(Context.NETWORK_STATS_SERVICE);
NetworkStats.Bucket bucket;
bucket = networkStatsManager.querySummaryForDevice(NetworkStats.Bucket.DEFAULT_NETWORK_ALL,"",startCalendar.getTimeInMillis(),stopCalendar.getTimeInMillis());
long rxBytes = bucket.getRxBytes();
long txBytes = bucket.getTxBytes();
我想获得每天的互联网使用情况。该应用程序可以访问数据使用，我还在清单中检查了所需的权限。
这是日志
2020-10-02 10:42:56.448 28091-28091/com.sreejithsreejayan.internetspeedometer D/AndroidRuntime: Shutting down VM
2020-10-02 10:42:56.452 28091-28091/com.sreejithsreejayan.internetspeedometer E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.sreejithsreejayan.internetspeedometer, PID: 28091
java.lang.NullPointerException: Attempt to invoke virtual method 'long android.app.usage.NetworkStats$Bucket.getRxBytes()' on a null object reference
at com.sreejithsreejayan.internetspeedometer.ui.home.HomeFragment$2.onClick(HomeFragment.java:71)
at android.view.View.performClick(View.java:7870)
at android.widget.TextView.performClick(TextView.java:14970)
at android.view.View.performClickInternal(View.java:7839)
at android.view.View.access$3600(View.java:886)
at android.view.View$PerformClick.run(View.java:29363)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:7814)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1068)
2020-10-02 10:42:56.490 28091-28091/com.sreejithsreejayan.internetspeedometer I/Process: Sending signal. PID: 28091 SIG: 9
为什么桶是空的（空）。我是android开发新手，请帮忙。