NPE: Thread Conversions - ConcurrentSkipListMap when storeMessageId

@ernir looks like there's a bug in your latest code change:

05-19 11:59:38.934 E/ACRA: ACRA caught a NullPointerException for org.briarproject.briar
                           java.lang.NullPointerException
                               at java.util.concurrent.ConcurrentSkipListMap.put(ConcurrentSkipListMap.java:1546)
                               at org.briarproject.bramble.api.data.BdfDictionary.of(BdfDictionary.java:26)
                               at org.briarproject.briar.client.MessageTrackerImpl.storeMessageId(MessageTrackerImpl.java:76)
                               at org.briarproject.briar.android.threaded.ThreadListControllerImpl$1.run(ThreadListControllerImpl.java:107)
                               at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
                               at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
                               at java.lang.Thread.run(Thread.java:761)