java JavaScriptException:（TypeError）：无法调用null的方法“getBoundingClientRect”

Question

在Errai/GWT应用程序中使用gwt-tour library时，会引发此错误：

com.google.gwt.core.client.JavaScriptException: (TypeError): Cannot call method 'getBoundingClientRect' of null at com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:248) 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.invokeNativeVoid(ModuleSpace.java:289) at com.google.gwt.dev.shell.JavaScriptHost.invokeNativeVoid(JavaScriptHost.java:107) at com.eemi.gwt.tour.client.GwtTour._createCallOut(GwtTour.java) at com.eemi.gwt.tour.client.GwtTour.createCallOut(GwtTour.java:106) at app.client.local.UIController$1.execute(UIController.java:46) at com.google.gwt.core.client.impl.SchedulerImpl$Task$.executeScheduled$(SchedulerImpl.java:50) at com.google.gwt.core.client.impl.SchedulerImpl.runScheduledTasks(SchedulerImpl.java:228) at com.google.gwt.core.client.impl.SchedulerImpl.flushPostEventPumpCommands(SchedulerImpl.java:388) at com.google.gwt.core.client.impl.SchedulerImpl$Flusher.execute(SchedulerImpl.java:78) at com.google.gwt.core.client.impl.SchedulerImpl.execute(SchedulerImpl.java:138) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) 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.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) 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)

这个错误意味着什么：

JavaScriptException: (TypeError): Cannot call method 'getBoundingClientRect' of null