Getting NullPointerException in GWT app but not sure how

Getting Uncaught exception escaped java.lang.NullPointerException: null

i am getting the error when calling

private final ObserverRegistrationImpl<Events.WordListEvent> wordListAdditionObservers =  new ObserverRegistrationImpl<Events.WordListEvent>();

public void addWordLists(ArrayList<WordList> wordLists) {
    for(WordList wl : wordLists) {
        GWT.log( "Model: Adding WordList: "+wl.getName());
        this.wordLists.add(wl);
                    //ERROR happens on the line below
        this.wordListAdditionObservers.notifyObservers(this, new Events.WordListEvent(wl)); 
    }
}

I checked in Debug mode and saw that wordListAdditionObservers, w1 and this were all objects. none were null. What could be wrong开发者_开发知识库?

Here is the ObserverRegistrationImpl class

public class ObserverRegistrationImpl<T> implements Observable<T> {

    private List<Observer<T>> observers = new ArrayList<Observer<T>>();


    @Override
    public void addObserver(Observer<T> o) {
        this.observers.add(o);
    }

    @Override
    public void removeAllObservers() {
        this.observers.clear();
    }

    @Override
    public void removeObserver(Observer<T> o) {
        this.observers.remove(o);
    }

    @Override
    public void notifyObservers(ModelViewInterface model, T event) {
        for(Observer<T> o : this.observers)
            o.notify(model, event);
    }

}

Here is the stacktrace

Uncaught exception escaped
<pre>java.lang.NullPointerException: null
    at com.example.gwt.myapplication.listeditor.client.ObserverRegistrationImpl.notifyObservers(ObserverRegistrationImpl.java:29)
    at com.example.gwt.myapplication.listeditor.client.Model.addWordLists(Model.java:103)
    at com.example.gwt.myapplication.listeditor.client.ControllerAuthentication$2.onXmlParsed(ControllerAuthentication.java:74)
    at com.example.gwt.myapplication.listeditor.client.ControllerAuthentication$2.onXmlParsed(ControllerAuthentication.java:1)
    at com.example.gwt.myapplication.listeditor.client.RemoteRequest.doParseXml(RemoteRequest.java:291)
    at com.example.gwt.myapplication.listeditor.client.RemoteRequest.access$0(RemoteRequest.java:282)
    at com.example.gwt.myapplication.listeditor.client.RemoteRequest$1.onResponseReceived(RemoteRequest.java:209)
    at com.google.gwt.http.client.Request.fireOnResponseReceived(Request.java:287)
    at com.google.gwt.http.client.RequestBuilder$1.onReadyStateChange(RequestBuilder.java:393)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
    at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
    at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:157)
    at com.google.gwt.dev.shell.BrowserChannel.reactToMessagesWhileWaitingForReturn(BrowserChannel.java:1714)
    at com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:165)
    at com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:120)
    at com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:507)
    at com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:264)
    at com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:91)
    at com.google.gwt.core.client.impl.Impl.apply(Impl.java)
    at com.google.gwt.core.client.impl.Impl.entry0(Impl.java:188)
    at sun.reflect.GeneratedMethodAccessor14.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
    at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
    at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:157)
    at com.google.gwt.dev.shell.BrowserChannel.reactToMessages(BrowserChannel.java:1669)
    at com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:401)
    at com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:222)
    at java.lang.Thread.run(Unknown Source)</pre>

I followed the problem in the debugger and noticed that it is thowing an exception at the following GWT code

} catch (InvocationTargetException e) {
  // If we get here, it means an exception is being thrown from
  // Java back into JavaScript
  wrapException(returnValue, e.getTargetException());
  return true;

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(Answered in comments. Converted to a community wiki answer. See Question with no answers, but issue solved in the comments (or extended in chat) )

The OP wrote:

It turns out the view getting passed to the controller was null because I was doing this private ControllerWordLists controller = new ControllerWordLists(this); in an instance variable. Now why is that possible? Why was there no compile error? That is just weird.

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I 've had the same stack trace. It should be double checked if there isn't any array of objects . GWT has problems with it. Better change it to List or List of Lists ...