DropDownModel not serializable

Opening a new browser window when a document editor with content blocks is open gives the following exception in the logs

ERROR [TP-Processor5] org.apache.wicket.markup.html.WebPage - Page [Page class = org.hippoecm.frontend.Home, id = 3, version = 0] couldn't be cloned to move to another pagemap
org.apache.wicket.WicketRuntimeException: Internal error cloning object
at org.apache.wicket.util.lang.Objects.cloneObject(Objects.java:550)
at org.apache.wicket.markup.html.WebPage.onNewBrowserWindow(WebPage.java:310)
...
Caused by: java.io.NotSerializableException: org.onehippo.forge.contentblocks.model.DropDownModel
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1509)