We could store all project files in the Gui by making a helper class that would extend ItemStack but would also contain the ProjectFile instance that way we could easily determine if the item is project file and we could easily filter pages until we find ItemStack that is instance of that helper class and we would also be able to get if that ItemStack is the main file.
We could store all project files in the Gui by making a helper class that would extend ItemStack but would also contain the ProjectFile instance that way we could easily determine if the item is project file and we could easily filter pages until we find ItemStack that is instance of that helper class and we would also be able to get if that ItemStack is the main file.