NPE during GWT compilation
<p>Following NPE appears during GWT compilation. It's in the compiler itself, not in application code:</p>

<pre><code>[ERROR] Exception in thread "pool-1-thread-96" java.lang.NullPointerException
[ERROR]     at com.google.gwt.dev.util.DiskCache.transferToStream(DiskCache.java:185)
[ERROR]     at com.google.gwt.dev.util.DiskCacheToken.writeObject(DiskCacheToken.java:73)
[ERROR]     at sun.reflect.GeneratedMethodAccessor29.invoke(Unknown Source)
[ERROR]     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[ERROR]     at java.lang.reflect.Method.invoke(Method.java:498)
[ERROR]     at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1028)
[ERROR]     at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
[ERROR]     at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
[ERROR]     at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
[ERROR]     at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
[ERROR]     at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:441)
[ERROR]     at com.google.gwt.dev.javac.CachedCompilationUnit.writeObject(CachedCompilationUnit.java:222)
[ERROR]     at sun.reflect.GeneratedMethodAccessor33.invoke(Unknown Source)
[ERROR]     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[ERROR]     at java.lang.reflect.Method.invoke(Method.java:498)
[ERROR]     at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1028)
[ERROR]     at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
[ERROR]     at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
[ERROR]     at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
[ERROR]     at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
[ERROR]     at com.google.gwt.dev.javac.PersistentUnitCache$6.run(PersistentUnitCache.java:492)
[ERROR]     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
[ERROR]     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
[ERROR]     at java.lang.Thread.run(Thread.java:745)
</code></pre>

<p>GWT version 2.6.1</p>