Crash due to icon URL being null.

This is quite a common ACRA crash report we are receiving for 0.103-alpha[1,2,3]:

java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.lastIndexOf(int)' on a null object reference
	at org.fdroid.fdroid.FDroidApp$5.generate(FDroidApp.java:292)
	at com.nostra13.universalimageloader.cache.disc.impl.BaseDiskCache.getFile(BaseDiskCache.java:167)
	at com.nostra13.universalimageloader.cache.disc.impl.BaseDiskCache.get(BaseDiskCache.java:98)
	at com.nostra13.universalimageloader.cache.disc.impl.LimitedAgeDiskCache.get(LimitedAgeDiskCache.java:74)
	at com.nostra13.universalimageloader.utils.DiskCacheUtils.findInCache(DiskCacheUtils.java:36)
	at org.fdroid.fdroid.NotificationHelper.getLargeIconForEntry(NotificationHelper.java:536)
	at org.fdroid.fdroid.NotificationHelper.createUpdateNotification(NotificationHelper.java:334)
	at org.fdroid.fdroid.NotificationHelper.createNotification(NotificationHelper.java:220)
	at org.fdroid.fdroid.NotificationHelper.access$400(NotificationHelper.java:38)
	at org.fdroid.fdroid.NotificationHelper$2.onReceive(NotificationHelper.java:139)
	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$1.handleMessage(LocalBroadcastManager.java:116)
	at android.os.Handler.dispatchMessage(Handler.java:102)
	at android.os.Looper.loop(Looper.java:154)
	at android.app.ActivityThread.main(ActivityThread.java:6126)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

(or some variation thereof, they all involve String.lastIndexOf() in FDroidApp.

assigned to @pserwylo

Addressed in the pending MR !490 (merged).

closed via merge request !490 (merged)