This was very probably introduced with the new CleanCacheService stuff.
Reported via ACRA:
ANDROID_VERSION=6.0.1
APP_VERSION_NAME=0.100-alpha6
STACK_TRACE=java.lang.NullPointerException: Attempt to get length of null array
at org.fdroid.fdroid.Utils.clearOldFiles(Utils.java:349)
at org.fdroid.fdroid.Utils.clearOldFiles(Utils.java:351)
at org.fdroid.fdroid.Utils.clearOldFiles(Utils.java:351)
at org.fdroid.fdroid.CleanCacheService.onHandleIntent(CleanCacheService.java:54)
at android.app.IntentService$ServiceHandler.handleMessage(IntentService.java:66)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.os.HandlerThread.run(HandlerThread.java:61)
There is nothing to clear if the directory could not be obtained for
some reason, so this seems like a reasonable fix. Anything is better
than a crash anyway.