This is more a question than a feature request / bug report.
In ContentHandler::getForModelID an exception is thrown in case that no content handler is registered. This might be the case, when an extension that introduced a new content model/handler gets disabled. In such a case one would need to change all invalid contentmodel entries in the database. Until this happens, the application may crash.
How about returning a TextContentHandler as fallback, or - if side effects could appear - some kind of UnkownContentHandler (which implements all required methods, but just doesn't output/alter anything)?