JavaScriptException during JSNI call: Invoking an instance method on a null instance

[ChrisK]

I'm trying to perform the following JSNI callback into Java in a GWT
2.0.3 project:

MyController (below) is initialised during module load. The app loads
fine but when I try to call js_callback from some plain JavaScript
code I get the below stacktrace. Any ideas?

private final Player player;

public MyController() {
    this.player = new PlayerImpl();
}

------------------------------------------------------------------------------

public PlayerImpl() {
    initialiseCallbacks();
}

private static native void initialiseCallbacks() /*-{
    $wnd.js_callback = $entry(th...@com.mine.playerimpl::callback());
}-*/;

------------------------------------------------------------------------------
And I get this when trying to call js_callback from some plain
JavaScript:
Uncaught exception: com.google.gwt.core.client.JavaScriptException:
(String): Invoking an instance method on a null instance
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native
Method)
    at
sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:
39)
    at
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:
27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at
com.google.gwt.dev.shell.ModuleSpace.createJavaScriptException(ModuleSpace.java:
65)
    at
com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:60)
    at
com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:
157)
    at
com.google.gwt.dev.shell.BrowserChannel.reactToMessagesWhileWaitingForReturn(BrowserChannel.java:
1713)
    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.GeneratedMethodAccessor158.invoke(Unknown Source)
    at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:
25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    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:
1668)
    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(Thread.java:619)

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