Will I be sinful if I continue to develop an application whose idea was stolen from a foreign application, after the head of the company declared that?

If the intention behind "stealing" an app idea is to benefit from it to create another app, then there is no harm in that. However, imitating the app without a fundamental addition constitutes an infringement on the owner's rights. It is permissible to work for a company that benefits from the idea without imitating it, but it is not permissible to work for a company that imitates the app and infringes upon the rights of others.