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java.lang.NullPointerException: null at java.util.Collections.sort(Collections.java:175)[na:1.8.0_60] at com.vaadin.data.util.ContainerHierarchicalWrapper.updateHierarchicalWrapper(ContainerHierarchicalWrapper.java:191)[vaadin-server-7.6.8.jar:7.6.8] at com.vaadin.data.util.ContainerHierarchicalWrapper$PiggybackListener.containerItemSetChange(ContainerHierarchicalWrapper.java:838)[vaadin-server-7.6.8.jar:7.6.8] at com.vaadin.data.util.AbstractContainer.fireItemSetChange(AbstractContainer.java:242)[vaadin-server-7.6.8.jar:7.6.8] at com.vaadin.data.util.AbstractContainer.fireItemSetChange(AbstractContainer.java:228)[vaadin-server-7.6.8.jar:7.6.8] at com.vaadin.data.util.ContainerHierarchicalWrapper.fireItemSetChangeIfAbstractContainer(ContainerHierarchicalWrapper.java:506)[vaadin-server-7.6.8.jar:7.6.8] at com.vaadin.data.util.ContainerHierarchicalWrapper.setParent(ContainerHierarchicalWrapper.java:495)[vaadin-server-7.6.8.jar:7.6.8] at com.xinra.listaide.frontend.EstablishRelationWindow.lambda$3(EstablishRelationWindow.java:104)[classes/:na] at java.lang.Iterable.forEach(Iterable.java:75)[na:1.8.0_60] at com.xinra.listaide.frontend.EstablishRelationWindow.buildChildrenTree(EstablishRelationWindow.java:102)[classes/:na]