Fixes a crash reported by ACRA:
java.lang.RuntimeException: Unable to destroy activity {org.fdroid.fdroid/org.fdroid.fdroid.AppDetails}: java.lang.NullPointerException
at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:3281)
at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:3299)
at android.app.ActivityThread.access$1200(ActivityThread.java:133)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4812)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:559)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at org.fdroid.fdroid.AppDetails.cleanUpFinishedDownload(AppDetails.java:442)
at org.fdroid.fdroid.AppDetails.onDestroy(AppDetails.java:567)
at android.app.Activity.performDestroy(Activity.java:5366)
at android.app.Instrumentation.callActivityOnDestroy(Instrumentation.java:1124)
at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:3268)
... 11 more