It looks like the context path was not available and calling length() causes this NPE?

org.zkoss.zk.ui.UiException: javax.servlet.ServletException: Unable to encode /zkau/view/z_yol/h5LQh/29f/1/c/a.png
at org.zkoss.zk.ui.http.ExecutionImpl.encodeURL(ExecutionImpl.java:328) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.impl.DesktopImpl.getUpdateURI(DesktopImpl.java:406) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.impl.DesktopImpl.getDynamicMediaURI(DesktopImpl.java:421) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zul.impl.Utils.getDynamicMediaURI(Utils.java:181) ~[zul-6.5.5.jar:6.5.5]
at org.zkoss.zul.Image.getEncodedURL(Image.java:230) ~[zul-6.5.5.jar:6.5.5]
at org.zkoss.zul.Image.renderProperties(Image.java:319) ~[zul-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.AbstractComponent.redraw(AbstractComponent.java:1807) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.AbstractComponent.redrawChildren(AbstractComponent.java:1884) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.AbstractComponent.redraw(AbstractComponent.java:1826) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.AbstractComponent.redrawChildren(AbstractComponent.java:1884) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.AbstractComponent.redraw(AbstractComponent.java:1826) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.AbstractComponent.redrawChildren(AbstractComponent.java:1884) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.AbstractComponent.redraw(AbstractComponent.java:1826) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.AbstractComponent.redrawChildren(AbstractComponent.java:1884) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.AbstractComponent.redraw(AbstractComponent.java:1826) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.impl.UiVisualizer.redraw(UiVisualizer.java:978) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.impl.UiVisualizer.getResponses(UiVisualizer.java:675) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.impl.UiEngineImpl.getResponses(UiEngineImpl.java:1420) [zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.impl.UiEngineImpl.endUpdate(UiEngineImpl.java:1126) [zk-6.5.5.jar:6.5.5]
at org.zkoss.zkex.ui.comet.CometServerPush.deactivate(CometServerPush.java:419) ~[zkex-6.5.5.jar:na]
at org.zkoss.zk.ui.impl.DesktopImpl.deactivateServerPush(DesktopImpl.java:1502) ~[zk-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.Executions.deactivate(Executions.java:980) ~[zk-6.5.5.jar:6.5.5]
Caused by: javax.servlet.ServletException: Unable to encode /zkau/view/z_yol/h5LQh/29f/1/c/a.png
at org.zkoss.web.servlet.http.Encodes.encodeURL(Encodes.java:405) ~[zweb-6.5.5.jar:6.5.5]
at org.zkoss.zk.ui.http.ExecutionImpl.encodeURL(ExecutionImpl.java:326) ~[zk-6.5.5.jar:6.5.5]
... 31 common frames omitted
Caused by: java.lang.NullPointerException: null
at org.zkoss.web.servlet.http.Encodes.encodeURL0(Encodes.java:494) ~[zweb-6.5.5.jar:6.5.5]
at org.zkoss.web.servlet.http.Encodes.access$000(Encodes.java:47) ~[zweb-6.5.5.jar:6.5.5]
at org.zkoss.web.servlet.http.Encodes$1.encodeURL(Encodes.java:429) ~[zweb-6.5.5.jar:6.5.5]
at org.zkoss.web.servlet.http.Encodes.encodeURL(Encodes.java:402) ~[zweb-6.5.5.jar:6.5.5]
... 32 common frames omitted