Editing a newly copied document type leads to NullPointerException

After having entered a new namespace with the /console I copied document type defaultcontent:article to praktijknu:article. When selecting the new node and clicking Edit model, the following exception occurs (after OK in the confirmation of course)

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An error has occurred, sorry for that.

java.lang.NullPointerException: null
Failed to instantiate plugin 'org.hippoecm.frontend.plugins.template.builder.PreviewFieldPlugin' for id 'org.hippoecm.frontend.plugin.PluginDescriptor@169eee8[
wicketId=field
className=org.hippoecm.frontend.plugins.template.builder.PreviewFieldPlugin
]'.