The issue is cause because you have a saved file that contains Pokemon that don't have a sprite.
A good practice is renaming your projects when you start a new one, else a clean project will now get stuck with a save file that crashes on load.
You can delete your saved file by going to the Saved Games folder on your computer.
Windows 10 has it kept here: C:\Users\Vendily-PC\Saved Games
Windows 7, it's a folder deeper, in Documents\Saved Games
Find the folder titled Pokemon Essentials, as that's the Project's title in the window bar, and delete Game.rxdata.
EDIT: Oh, and you'll need sprites for the new pokemon too. They can be dummy or placeholder sprites, you just need something.