An exception occurs when using ace:dataTable and a header columnGroup, and trying to sort.

From the server log we can see that ColumnBase is trying to state save a Map, which has a null value in it, that's causing the exception. Since column has no Map properties, and the stack trace isn't showing much saveState recursion, it's likely that it's the StateHelper's own Map that has the null entry.

java.lang.NullPointerException
at javax.faces.component.StateHolderSaver.<init>(StateHolderSaver.java:96)
at javax.faces.component.UIComponentBase.saveAttachedState(UIComponentBase.java:1695)
at javax.faces.component.ComponentStateHelper.saveMap(ComponentStateHelper.java:378)
at javax.faces.component.ComponentStateHelper.saveState(ComponentStateHelper.java:256)
at javax.faces.component.UIComponentBase.saveState(UIComponentBase.java:1501)
at org.icefaces.ace.component.column.ColumnBase.saveState(ColumnBase.java:1324)
at com.sun.faces.application.view.StateManagementStrategyImpl$1.visit(StateManagementStrategyImpl.java:168)
at com.sun.faces.component.visit.FullVisitContext.invokeVisitCallback(FullVisitContext.java:151)
at org.icefaces.ace.component.datatable.DataTable.visitColumnsAndColumnFacets(DataTable.java:502)
at org.icefaces.ace.component.datatable.DataTable.visitTree(DataTable.java:466)
at javax.faces.component.UIComponent.visitTree(UIComponent.java:1600)
at javax.faces.component.UIForm.visitTree(UIForm.java:344)
at javax.faces.component.UIComponent.visitTree(UIComponent.java:1600)
at javax.faces.component.UIComponent.visitTree(UIComponent.java:1600)
at com.sun.faces.application.view.StateManagementStrategyImpl.saveView(StateManagementStrategyImpl.java:160)
at com.sun.faces.application.StateManagerImpl.saveView(StateManagerImpl.java:133)
at javax.faces.application.StateManager.getViewState(StateManager.java:553)
at org.icefaces.impl.context.DOMPartialViewContext.renderState(DOMPartialViewContext.java:448)
at org.icefaces.impl.context.DOMPartialViewContext.processPartial(DOMPartialViewContext.java:205)
at javax.faces.component.UIViewRoot.encodeChildren(UIViewRoot.java:981)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1756)
at com.sun.faces.application.view.FaceletViewHandlingStrategy.renderView(FaceletViewHandlingStrategy.java:390)
at com.sun.faces.application.view.MultiViewHandler.renderView(MultiViewHandler.java:131)
at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:121)
at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:139)
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:594)