Code:

@Nullable EntityEquipment entityEquipment = entity.getEquipment();
if (entityEquipment == null) return;

/* .. apply equipment via entityEquipment.setItem(slot, itemToSet); */ 

Might be that we need to handle mannequin here: https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/src/main/java/org/bukkit/craftbukkit/entity/CraftLivingEntity.java#119

Or just always return the underlying equipment of Minecraft's LivingEntity? I am not sure why Bukkit's LivingEntity.getEquipment is nullable in the first place (I don't think there is any LivingEntity subtype that does not return something for getEquipment, except for mannequin currently)