This is now fixed for item templates.

For index templates we can still get tracebacks, e.g., when we have an 'id' field or a Link property that links to a Class that has no key property. For these we could enter something non-numeric or simply a number that is too big for the database backend's integer. These produce a traceback and there may be others.

The real fix would be to catch errors from the database backend and add them to the displayed error message with client.add_error_message(). Unfortunately the current index templates do not display error messages.