The solution is really to get rid of the Fragment and do everything in the
Activity, especially since this nullguard will probably leave things in a
not good state. But that's better than a crash, I think.
W java.lang.NullPointerException
W at org.fdroid.fdroid.views.fragments.AvailableAppsFragment$CategoryObserver.onChange(AvailableAppsFragment.java:88)
W at org.fdroid.fdroid.views.fragments.AvailableAppsFragment$CategoryObserver.onChange(AvailableAppsFragment.java:103)
W at android.database.ContentObserver.dispatchChange(ContentObserver.java:163)
W at android.database.ContentObserver$Transport.onChange(ContentObserver.java:195)
W at android.database.IContentObserver$Stub.onTransact(IContentObserver.java:60)
W at android.os.Binder.execTransact(Binder.java:404)
W at dalvik.system.NativeStart.run(Native Method)