There is no such file, indeed (i.e. "BlueSky.png"). But AFAIK, this file is never called in the current scripts of the game (i.e. versions 1.90 and 1.91).
So IMHO, the question would be instead: how do you get this error ?
Have you downloaded the -latest- version indicated in the first post of this thread ? Are you using another version (for instance a "compressed" one, or may be the unofficial android port also available in the first post, or ...) ?
Have you installed the new version in the same directory/folder as a previous one ? You should try a clean install in that case (do not forget to save your saved game files before, and to reinstall them after).