Adds some core behavior code, uses it to fix a small bug

code/modules/client/preference_setup/traits/trait_defines.dm

@@ -2,6 +2,7 @@
 // The actual modifiers (if used) for these are stored inside code/modules/mob/_modifiers/traits.dm
 
 /datum/trait/modifier
+	abstract_type = /datum/trait/modifier
 	var/modifier_type = null // Type to add to the mob post spawn.
 
 /datum/trait/modifier/apply_trait_post_spawn(mob/living/L)
@@ -23,6 +24,7 @@
 
 // Physical traits are what they sound like, and involve the character's physical body, as opposed to their mental state.
 /datum/trait/modifier/physical
+	abstract_type = /datum/trait/modifier/physical
 	name = "Physical"
 	category = "Physical"
 
@@ -195,6 +197,7 @@
 // 'Mental' traits are just those that only sapients can have, for now, and generally involves fears.
 // So far, all of them are just for fluff/don't have mechanical effects.
 /datum/trait/modifier/mental
+	abstract_type = /datum/trait/modifier/mental
 	name = "Mental"
 	category = "Mental"
 

code/modules/client/preference_setup/traits/traits.dm

@@ -5,7 +5,9 @@ var/list/trait_categories = list() // The categories available for the trait men
 /hook/startup/proc/populate_trait_list()
 
 	//create a list of trait datums
-	for(var/trait_type in typesof(/datum/trait) - list(/datum/trait, /datum/trait/modifier))
+	for(var/trait_type as anything in subtypesof(/datum/trait))
+		if (is_abstract(trait_type))
+			continue
 		var/datum/trait/T = new trait_type
 
 		if(!T.name)
@@ -141,6 +143,7 @@ var/list/trait_categories = list() // The categories available for the trait men
 
 
 /datum/trait
+	abstract_type = /datum/trait
 	var/name = null							// Name to show on UI
 	var/desc = null							// Description of what it does, also shown on UI.
 	var/list/mutually_exclusive = list()	// List of trait types which cannot be taken alongside this trait.