Found in GWT Release 1.7
Encountered on Win Vista / IE7, FireFox 3, Chrome

Description:
I encountered a situation where code works fine in hosted mode but fails
when compiled and ran from a real browser.  I subclassed an array list to
add change notification.  In the constructor that accepts a Collection<>, I
initalize an ArrayList of _listeners.  

When running in Java, this code compiles and runs fine.  When compiled to
javascript, I get a null error.  The problem is that the ArrayList
implementation in GWT appears to call on an inner method to perform the
add.  This method attempts to check the _listeners collection.  Since the
_listener collection was null, it failed.  My code was easy to fix, but it
was a frustrating bug to deal with since it worked fine in hosted mode.


Sample Code:
public class ObservableList<E> extends ArrayList<E>{

   ArrayList<Listener<BaseEvent>> _listeners;
   
   public ObservableList(Collection<? extends E> collection){
       super(collection);
       _listeners = new ArrayList<Listener<BaseEvent>>();
   }

   ...
   
   public boolean addAll(Collection<? extends E> arg0){
        boolean r = super.addAll(arg0);
        onCollectionChange(arg0);
        return r;
   }

   ...
   
   private void onCollectionChange(Collection arg0){
       for(int i=0; i<_listeners.size(); i++)
          ...  
       //GWT hits this code after constructor.  Java does not.
   }


Workaround:
check for _listeners == null in the onCollectionChange method