That is a legitimate concern. Keep in mind that you can request non-publication, if you do not wish to pursue foreign patents, in which case your application would not be made public before it is granted (which takes about 3 years at a minimum). Also note that even if someone has a patent for an improved version, they still would not be able to sell it if it infringes your patent. A patent does not give you an affirmative right to sell something, it only gives you a right to stop other people.