When a wiki is created it doesn't have any subwiki. The mod_wiki_get_subwikis WS returns id -1 for those subwikis that aren't created yet.
The WebService mod_wiki_new_page requires a subwikiid to work. This means that in external services like the Mobile app a user won't be able to create new pages if the subwiki isn't created yet. So in the end the user is unable to do anything until he views the wiki in Moodle web.
We should make mod_wiki_new_page accept a combination of userid+groupid as an alternative for the subwikiid, as it's done in other WebServices like mod_wiki_get_subwiki_pages.