This is very related to #660 but this time, I can't see any other way to
solve it but a null guard. I don't think it is possible to guarantee that
the Downloader.ACTION_INTERRUPTED receiver will be unregistered since
onDestroy() might not even be called.
java.lang.NullPointerException
at org.fdroid.fdroid.installer.InstallManagerService.removeFromActive(InstallManagerService.java:328)
at org.fdroid.fdroid.installer.InstallManagerService.access$400(InstallManagerService.java:58)
at org.fdroid.fdroid.installer.InstallManagerService$4.onReceive(InstallManagerService.java:212)
at android.support.v4.content.LocalBroadcastManager.executePendingBroadcasts(LocalBroadcastManager.java:297)
at android.support.v4.content.LocalBroadcastManager.access$000(LocalBroadcastManager.java:46)
at android.support.v4.content.LocalBroadcastManager$1.handleMessage(LocalBroadcastManager.java:116)
at android.os.Handler.dispatchMessage(Handler.java:110)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:5353)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:830)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:646)
at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
at dalvik.system.NativeStart.main(Native Method)
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