I attached the .7z you sent me to the post. Don't know why it worked for me?
I figured out what the problem was.
It's a bit tricky. If you take out MainProg.ec of the project, and then add it back at the end of the project, the bush will display fine.
The problem is that the MainFrame program was being instanciated before the tilesetGfx, and so when doing res = tilesetGfx, res was null.
Thus the order in which instances are created became important... Ideally the order of files in the project shouldn't matter, but this is still something I need to improve

I found this out by putting a breakpoint in TileObject::Render and looking at the value of 'bmp' and 'res' (null).
Another way would be to move the program instance after tilesetGfx in game.ec (You would need to add import "MainProg" at the top), or move tilesetGfx before program in MainProg.ec (You would need to add import "Game" at the top).
Regards,
Jerome