I've just tried the brix-demo master branch.
When I click "Edit" for a reference in the main menu, I get a wicket runtimeexception on the nodePicker caused by:
Caused by: java.lang.IllegalStateException: ITargetRespondListeners can no longer be added
at org.apache.wicket.ajax.AjaxRequestTarget.assertNotFrozen(AjaxRequestTarget.java:309)
at org.apache.wicket.ajax.AjaxRequestTarget.assertRespondersNotFrozen(AjaxRequestTarget.java:326)
at org.apache.wicket.ajax.AjaxRequestTarget.registerRespondListener(AjaxRequestTarget.java:599)
at org.apache.wicket.markup.html.tree.AbstractTree.updateTree(AbstractTree.java:1139)
at com.inmethod.grid.treegrid.TreeGrid.update(TreeGrid.java:256)
at org.brixcms.web.picker.common.NodePickerTreeGridPanel$2.onItemSelectionChanged(NodePickerTreeGridPanel.java:101)
at com.inmethod.grid.treegrid.TreeGrid.access$300(TreeGrid.java:34)
at com.inmethod.grid.treegrid.TreeGrid$TreeStateListener.nodeSelected(TreeGrid.java:201)
at org.apache.wicket.markup.html.tree.DefaultTreeState.selectNode(DefaultTreeState.java:231)
at org.apache.wicket.markup.html.tree.DefaultTreeState.selectNode(DefaultTreeState.java:250)
at com.inmethod.grid.treegrid.TreeGrid$2.selectNode(TreeGrid.java:139)
at com.inmethod.grid.treegrid.TreeGrid.selectItem(TreeGrid.java:386)
at org.brixcms.web.picker.node.NodePicker.updateSelection(NodePicker.java:71)
at org.brixcms.web.picker.node.NodePicker.access$1(NodePicker.java:62)
at org.brixcms.web.picker.node.NodePicker$1.configureGrid(NodePicker.java:46)
at org.brixcms.web.picker.common.NodePickerTreeGridPanel.initComponents(NodePickerTreeGridPanel.java:129)
at org.brixcms.web.picker.common.NodePickerTreeGridPanel.onBeforeRender(NodePickerTreeGridPanel.java:84)
at org.apache.wicket.Component.internalBeforeRender(Component.java:981)
at org.apache.wicket.Component.beforeRender(Component.java:1015)
at org.apache.wicket.MarkupContainer.onBeforeRenderChildren(MarkupContainer.java:1785)
I certainly hope this is due to an error on my behalf, although all I've done is downloaded the github code and started the demo.