I've had an offline discussion with Jesse on the
use of ErrorEvent in the book drafts:

>>>- p.4, 5: Why is last arg of ErrorEvent ""
instead of null?
>>Javadoc does not say it may be null, so it
cannot be null.
> Well, I use it that way in my module based on
some advice 
> on the newsgroup a long time ago from Ales
Novak.  So, 
> either the javadoc (and this example) should be
updated or 
> my code (and whatever example Ales had probably
in 
> openide and core) should be updated.

Probably the latter. Note that as far as I know,
passing null is 
harmless. But if it winds up throwing an NPE
somewhere, that's your problem. Permitting it to
accept null in the constructor (i.e. changing
Javadoc) is technically incompatible because that
would imply the getReferenceText() method could
now return null, and existing clients may not
check for it. I don't see any particular need to
let it accept  null, other than that stylistically
that would be more natural.

So, as as result, I've done a search and found
this used in:
java/src/org/netbeans/modules/java/BogusCompiler.java
java/src/org/netbeans/modules/java/CleanCompilerGroup.java
java/src/org/netbeans/modules/java/CleanCompiler.java
java/src/org/netbeans/modules/java/JExternalCompilerGroup.java