If you start Ametys with an existing content, with no corresponding content type (plugin removed for example). You get an error when you try to view this content:

Caused by: org.ametys.runtime.model.exception.UndefinedItemPathException: Unable to retrieve the value at path 'title'. There is no such item defined by the model.
at org.ametys.plugins.repository.data.holder.impl.DefaultModelAwareDataHolder._getValue(DefaultModelAwareDataHolder.java:183)
at org.ametys.plugins.repository.data.holder.impl.DefaultModelAwareDataHolder.getValue(DefaultModelAwareDataHolder.java:165)

It will be best to log that the content type is unknown.