Regression: Editing a taxonomy no longer works - saving fails and after closing window the taxonomy no longer can be opened

Reproduction path:

• Navigate to the Taxonomies in the document perspective
• select a taxonomy (e.g. "en")
• click "Edit"
• click "Save & Close"
• nothing happens
• close the window manually (using the window 'x' action)
• try opening the same taxonomy again
• nothing happens

Note, even after a restart, the taxonomy no longer can be opened.

In the Console the following stacktrace is shown:

ERROR [org.apache.wicket.DefaultExceptionMapper.internalMap():123] Unexpected error occurred
org.apache.wicket.WicketRuntimeException: Error attaching this container for rendering: [TaxonomyEditorPlugin [Component id = template]]
at org.apache.wicket.MarkupContainer.onBeforeRenderChildren(MarkupContainer.java:1693)
at org.apache.wicket.Component.onBeforeRender(Component.java:3843)
...
Caused by: java.lang.NullPointerException
at org.onehippo.taxonomy.plugin.tree.CategoryNode.<init>(CategoryNode.java:35)
at org.onehippo.taxonomy.plugin.tree.AbstractNode.getChildren(AbstractNode.java:58)
at org.onehippo.taxonomy.plugin.tree.AbstractNode.getChildCount(AbstractNode.java:85)
at javax.swing.tree.DefaultTreeModel.getChildCount(DefaultTreeModel.java:186)
at org.apache.wicket.extensions.markup.html.tree.AbstractTree.nodeChildren(AbstractTree.java:1614)
at org.apache.wicket.extensions.markup.html.tree.AbstractTree.buildItemChildren(AbstractTree.java:1234)
at org.apache.wicket.extensions.markup.html.tree.AbstractTree.onBeforeRenderInternal(AbstractTree.java:644)
at org.apache.wicket.extensions.markup.html.tree.AbstractTree.onBeforeRender(AbstractTree.java:659)
at org.apache.wicket.Component.internalBeforeRender(Component.java:922)
at org.apache.wicket.Component.beforeRender(Component.java:990)
at org.apache.wicket.MarkupContainer.onBeforeRenderChildren(MarkupContainer.java:1681)
... 54 more