SEVERE: 
org.zkoss.zk.ui.UiException: javax.servlet.ServletException: Unable to encode ~./some/resource.png
	at org.zkoss.zk.ui.http.ExecutionImpl.encodeURL(ExecutionImpl.java:323)
	at org.zkoss.zk.ui.Components$Exec.encodeURL(Components.java:790)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at bsh.Reflect.invokeMethod(Reflect.java:131)
	at bsh.Reflect.invokeObjectMethod(Reflect.java:77)
	at bsh.Name.invokeMethod(Name.java:852)
	at bsh.BSHMethodInvocation.eval(BSHMethodInvocation.java:69)
	at bsh.BSHPrimaryExpression.eval(BSHPrimaryExpression.java:96)
	at bsh.BSHPrimaryExpression.eval(BSHPrimaryExpression.java:41)
	at bsh.Interpreter.eval(Interpreter.java:659)
	at bsh.Interpreter.eval(Interpreter.java:750)
	at org.zkoss.zk.scripting.bsh.BSHInterpreter.exec(BSHInterpreter.java:131)
	at org.zkoss.zk.scripting.util.GenericInterpreter.interpret(GenericInterpreter.java:342)
	at org.zkoss.zk.ui.impl.PageImpl.interpret(PageImpl.java:965)
	at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:3120)
	at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:3074)
	at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:138)
	at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1846)
	at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1618)
	at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1321)
	at org.zkoss.zkmax.au.websocket.WebSocketEndPoint.process(WebSocketEndPoint.java:228)
	at org.zkoss.zkmax.au.websocket.WebSocketEndPoint.access$000(WebSocketEndPoint.java:56)
	at org.zkoss.zkmax.au.websocket.WebSocketEndPoint$1.onMessage(WebSocketEndPoint.java:93)
	at org.zkoss.zkmax.au.websocket.WebSocketEndPoint$1.onMessage(WebSocketEndPoint.java:90)
	at org.eclipse.jetty.websocket.jsr356.messages.TextWholeMessage.messageComplete(TextWholeMessage.java:56)
	at org.eclipse.jetty.websocket.jsr356.endpoints.JsrEndpointEventDriver.onTextFrame(JsrEndpointEventDriver.java:218)
	at org.eclipse.jetty.websocket.common.events.AbstractEventDriver.incomingFrame(AbstractEventDriver.java:162)
	at org.eclipse.jetty.websocket.common.WebSocketSession.incomingFrame(WebSocketSession.java:375)
	at org.eclipse.jetty.websocket.common.extensions.AbstractExtension.nextIncomingFrame(AbstractExtension.java:182)
	at org.eclipse.jetty.websocket.common.extensions.compress.PerMessageDeflateExtension.nextIncomingFrame(PerMessageDeflateExtension.java:105)
	at org.eclipse.jetty.websocket.common.extensions.compress.CompressExtension.forwardIncoming(CompressExtension.java:142)
	at org.eclipse.jetty.websocket.common.extensions.compress.PerMessageDeflateExtension.incomingFrame(PerMessageDeflateExtension.java:85)
	at org.eclipse.jetty.websocket.common.extensions.ExtensionStack.incomingFrame(ExtensionStack.java:220)
	at org.eclipse.jetty.websocket.common.Parser.notifyFrame(Parser.java:220)
	at org.eclipse.jetty.websocket.common.Parser.parse(Parser.java:256)
	at org.eclipse.jetty.websocket.common.io.AbstractWebSocketConnection.readParse(AbstractWebSocketConnection.java:679)
	at org.eclipse.jetty.websocket.common.io.AbstractWebSocketConnection.onFillable(AbstractWebSocketConnection.java:511)
	at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:279)
	at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:110)
	at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:124)
	at org.eclipse.jetty.util.thread.Invocable.invokePreferred(Invocable.java:128)
	at org.eclipse.jetty.util.thread.Invocable$InvocableExecutor.invoke(Invocable.java:222)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:294)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:199)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:673)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:591)
	at java.lang.Thread.run(Thread.java:748)
Caused by: javax.servlet.ServletException: Unable to encode ~./some/resource.png
	at org.zkoss.web.servlet.http.Encodes.encodeURL(Encodes.java:402)
	at org.zkoss.zk.ui.http.ExecutionImpl.encodeURL(ExecutionImpl.java:321)
	... 49 more
Caused by: java.lang.NullPointerException
	at org.zkoss.web.servlet.Servlets.getExtWebCtxs(Servlets.java:1320)
	at org.zkoss.web.servlet.Servlets.getExtendletContext(Servlets.java:1312)
	at org.zkoss.web.servlet.http.Encodes.encodeURL0(Encodes.java:453)
	at org.zkoss.web.servlet.http.Encodes.access$000(Encodes.java:48)
	at org.zkoss.web.servlet.http.Encodes$1.encodeURL(Encodes.java:426)
	at org.zkoss.web.servlet.http.Encodes.encodeURL(Encodes.java:399)
	... 50 more