I was doing some testing using GXT 3.0.2 with IE9 (in compatibility mode) and I got this error when I was typing text in a GXT TextField. I think it fired as I typed the third character and the cursor was still in the field (but could have been on the blur event).

java.lang.AssertionError: Child cannot be null: at
com.google.gwt.dom.client.Node$.isOrHasChild$(Node.java:278)
com.sencha.gxt.cell.core.client.form.FieldCell.onBrowserEvent(FieldCell.java:264)
com.sencha.gxt.cell.core.client.form.TextInputCell.onBrowserEvent(TextInputCell.java:88)
com.sencha.gxt.cell.core.client.form.TextInputCell.onBrowserEvent(TextInputCell.java:1)
com.sencha.gxt.widget.core.client.cell.CellComponent.onBrowserEvent(CellComponent.java:221)
com.sencha.gxt.widget.core.client.form.Field.onBrowserEvent(Field.java:318)
com.google.gwt.user.client.DOM.dispatchEventImpl(DOM.java:1351)
com.google.gwt.user.client.DOM.dispatchEvent(DOM.java:1307)
sun.reflect.GeneratedMethodAccessor44.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:601)
com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:337)
com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:218)
com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:136)
com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:561)
com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:269)
com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:91)
com.google.gwt.core.client.impl.Impl.apply(Impl.java)
...
java.lang.AssertionError: Child cannot be null
at com.google.gwt.dom.client.Node$.isOrHasChild$(Node.java:278)
at com.sencha.gxt.cell.core.client.form.FieldCell.onBrowserEvent(FieldCell.java:264)
at com.sencha.gxt.cell.core.client.form.TextInputCell.onBrowserEvent(TextInputCell.java:88)
at com.sencha.gxt.cell.core.client.form.TextInputCell.onBrowserEvent(TextInputCell.java:1)
at com.sencha.gxt.widget.core.client.cell.CellComponent.onBrowserEvent(CellComponent.java:221)
at com.sencha.gxt.widget.core.client.form.Field.onBrowserEvent(Field.java:318)
at com.google.gwt.user.client.DOM.dispatchEventImpl(DOM.java:1351)
at com.google.gwt.user.client.DOM.dispatchEvent(DOM.java:1307)
at sun.reflect.GeneratedMethodAccessor44.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
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.GeneratedMethodAccessor38.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
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(Thread.java:722)
13 Nov 2012 15:15:59,548 ERROR GWTDashboardEntryPoint []: CLIENT MSG: Child cannot be null
java.lang.AssertionError: Child cannot be null: at
com.google.gwt.dom.client.Node$.isOrHasChild$(Node.java:278)
com.sencha.gxt.cell.core.client.form.FieldCell.onBrowserEvent(FieldCell.java:264)
com.sencha.gxt.cell.core.client.form.TextInputCell.onBrowserEvent(TextInputCell.java:88)
com.sencha.gxt.cell.core.client.form.TextInputCell.onBrowserEvent(TextInputCell.java:1)
com.sencha.gxt.widget.core.client.cell.CellComponent.onBrowserEvent(CellComponent.java:221)
com.sencha.gxt.widget.core.client.form.Field.onBrowserEvent(Field.java:318)
com.google.gwt.user.client.DOM.dispatchEventImpl(DOM.java:1351)
com.google.gwt.user.client.DOM.dispatchEvent(DOM.java:1307)
sun.reflect.GeneratedMethodAccessor44.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:601)
com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:337)
com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:218)
com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:136)
com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:561)
com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:269)
com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:91)
com.google.gwt.core.client.impl.Impl.apply(Impl.java)
...