coa.16: Talents view no longer crashes on CoA custom classes

DataStore_Talents._GetTreeReference degrades to nil (was assert) when a custom class
(MONK, etc.) has no/partial talent reference data; Talents.lua skips background render
when GetTreeInfo returns no bg.

DataStore_Talents/DataStore_Talents.lua

@@ -239,9 +239,12 @@ local function	_GetClassReference(class)
 end
 
 local function _GetTreeReference(class, tree)
-	assert(type(class) == "string")
-	assert(type(tree) == "string")
-	return addon.ref.global[class].Trees[tree]
+	-- CoA: custom classes (MONK, etc.) may have no/partial talent reference data, so a
+	-- tree lookup can arrive with a nil tree name. Degrade to nil instead of asserting.
+	if type(class) ~= "string" or type(tree) ~= "string" then return end
+	local c = addon.ref.global[class]
+	if not c or not c.Trees then return end
+	return c.Trees[tree]
 end
 
 local function _IsClassKnown(class)