ItemInitializer.java:129: error: incompatible types: ItemStack cannot be converted to ResourceLocation
GameRegistry.addShapedRecipe(new ItemStack(gardenStaff), " x", " / ", "/ ", 'x', Items.FLOWER_POT, '/',

Recipes in 1.12.2 are registered using JSON instead of using GameRegistry.addShapedRecipe(..).

Links:

https://www.minecraftforum.net/forums/mapping-and-modding-java-edition/minecraft-mods/modification-development/2836442-mc-1-12-modding-does-anyone-know-how-to-add
https://github.com/skylinerw/guides/blob/master/java/recipes.md

Examples:

https://github.com/cleverpanda/SimpleCorn/tree/1.12/src/main/resources/assets/simplecorn/recipes

Closed with #286 and commit b6fd0d2.