when many crawl queues are generated, this NPE can occur; probably

caused as concurrency issue:
W 2015/09/05 14:09:10 ConcurrentLog java.lang.NullPointerException
java.lang.NullPointerException
	at java.util.TreeMap.rotateRight(TreeMap.java:2239)
	at java.util.TreeMap.fixAfterInsertion(TreeMap.java:2271)
	at java.util.TreeMap.put(TreeMap.java:582)
	at net.yacy.kelondro.table.Table.<init>(Table.java:235)
	at net.yacy.crawler.HostQueue.openStack(HostQueue.java:229)
	at net.yacy.crawler.HostQueue.getStack(HostQueue.java:204)
	at net.yacy.crawler.HostQueue.push(HostQueue.java:397)
	at net.yacy.crawler.HostBalancer.push(HostBalancer.java:237)
	at net.yacy.crawler.data.NoticedURL.push(NoticedURL.java:184)
	at net.yacy.crawler.CrawlStacker.stackCrawl(CrawlStacker.java:355)
	at net.yacy.crawler.CrawlStacker.job(CrawlStacker.java:134)
	at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
	at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at
net.yacy.kelondro.workflow.InstantBlockingThread.job(InstantBlockingThread.java:101)
	at
net.yacy.kelondro.workflow.AbstractBlockingThread.run(AbstractBlockingThread.java:82)
	at
java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)

source/net/yacy/kelondro/table/Table.java

@@ -232,7 +232,7 @@ public class Table implements Index, Iterable<Row.Entry> {
         }
 
         // track this table
-        tableTracker.put(tablefile.toString(), this);
+        synchronized (tableTracker) {tableTracker.put(tablefile.toString(), this);}
     }
 
     public synchronized void warmUp() {