Minor Description
As explained by a user at http://markmail.org/message/s262q4yc3ubymglr it is unexpected that org.apache.wicket.markup.html.form.Form#findSubmittingButton() throws runtime exceptions when the submitter is invisible or disabled.
As discussed at http://markmail.org/message/qhjepkihr27uywaj the checks should be moved to the code related to executing the submitter's action method (i.e. onSubmit()) and leave all other users of Form#findSubmittingButton() decide for themselves whether the change of the visiblility/enable state is a problem of not.