How do I troubleshoot "Something other than a Java object was returned from JSNI method" error?

Hi,

I'm using GWT 2.4. Occassionally, I'll get errors like the below --
"Something other than a Java object was returned from JSNI method".
In this case, the closest line in my code (line 366) is

String type = nativeEvent.getType();

where "nativeEvent" is of type
"com.google.gwt.dom.client.NativeEvent". How do I figure out what
"event" GWT is complaining about? I cannot consistently reproduce
this error, but I notice it in my development mode logs every so
often.

Thanks, - Dave

08:09:05.779 [ERROR] [productplus] Uncaught exception escaped

com.google.gwt.event.shared.UmbrellaException: One or more exceptions
caught, see full set in UmbrellaException#getCauses
at
com.google.gwt.event.shared.HandlerManager.fireEvent(HandlerManager.java:
129)
at com.google.gwt.user.client.Event
$NativePreviewEvent.fire(Event.java:87)
at com.google.gwt.user.client.Event$NativePreviewEvent.access
$4(Event.java:73)
at com.google.gwt.user.client.Event
$.fireNativePreviewEvent(Event.java:488)
at com.google.gwt.user.client.DOM.previewEvent(DOM.java:1328)
at sun.reflect.GeneratedMethodAccessor30.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:
172)
at
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:
337)
at
com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:
218)
at
com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:
136)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:
561)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:
269)
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:213)
at sun.reflect.GeneratedMethodAccessor28.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:
172)
at
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:
337)
at
com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:
218)
at
com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:
136)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:
561)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:
269)
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:213)
at sun.reflect.GeneratedMethodAccessor28.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:
172)
at
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessages(BrowserChannelServer.java:
292)
at
com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:
546)
at
com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:
363)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.IllegalArgumentException: Something other than a
Java object was returned from JSNI method
'@com.google.gwt.dom.client.DOMImpl::eventGetType(Lcom/google/gwt/dom/
client/NativeEvent;)': JS value of type boolean, expected
java.lang.Object
at com.google.gwt.dev.shell.JsValueGlue.get(JsValueGlue.java:178)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:
271)
at
com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:
91)
at com.google.gwt.dom.client.DOMImpl.eventGetType(DOMImpl.java)
at com.google.gwt.dom.client.NativeEvent$.getType$
(NativeEvent.java:268)
at com.myco.clearing.product.client.widget.DndController
$DragEventsPreviewHandler.onPreviewNativeEvent(DndController.java:366)
at com.google.gwt.user.client.Event
$NativePreviewEvent.dispatch(Event.java:195)
at com.google.gwt.user.client.Event
$NativePreviewEvent.dispatch(Event.java:1)
at com.google.gwt.event.shared.GwtEvent.dispatch(GwtEvent.java:1)
at
com.google.web.bindery.event.shared.EventBus.dispatchEvent(EventBus.java:
40)
at
com.google.web.bindery.event.shared.SimpleEventBus.doFire(SimpleEventBus.java:
193)
at
com.google.web.bindery.event.shared.SimpleEventBus.fireEvent(SimpleEventBus.java:
88)
at
com.google.gwt.event.shared.HandlerManager.fireEvent(HandlerManager.java:
127)
at com.google.gwt.user.client.Event
$NativePreviewEvent.fire(Event.java:87)
at com.google.gwt.user.client.Event$NativePreviewEvent.access
$4(Event.java:73)
at com.google.gwt.user.client.Event
$.fireNativePreviewEvent(Event.java:488)
at com.google.gwt.user.client.DOM.previewEvent(DOM.java:1328)
at sun.reflect.GeneratedMethodAccessor30.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:
172)
at
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:
337)
at
com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:
218)
at
com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:
136)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:
561)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:
269)
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:213)
at sun.reflect.GeneratedMethodAccessor28.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:
172)
at
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:
337)
at
com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:
218)
at
com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:
136)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:
561)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:
269)
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:213)
at sun.reflect.GeneratedMethodAccessor28.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:
172)
at
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessages(BrowserChannelServer.java:
292)
at
com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:
546)
at
com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:
363)
at java.lang.Thread.run(Unknown Source)

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