Fix #1858 - crash when adding a modifier to a jewel

This is the same issue as was fixed in abyss jewels in 9d42348. If there
are no options in the crafting table, the "crafting table" option needs
to be hidden, or there will be an error when trying to create the
tooltip.

For good measure, this patch also prevents the crash by checking that
there's something in listMod before dereferencing it.

Author: dcollinsn

Classes/ItemsTab.lua

@@ -1613,7 +1613,7 @@ function ItemsTabClass:AddCustomModifierToDisplayItem()
 			end)
 		end
 	end
-	if (self.build.targetVersion ~= "2_6" and self.displayItem.base.subType ~= "Abyss") or (self.displayItem.type ~= "Jewel" and self.displayItem.type ~= "Flask") then
+	if (self.build.targetVersion ~= "2_6" and self.displayItem.type ~= "Jewel") or (self.displayItem.type ~= "Jewel" and self.displayItem.type ~= "Flask") then
 		t_insert(sourceList, { label = "Crafting Bench", sourceId = "MASTER" })
 	end
 	if self.displayItem.type ~= "Jewel" and self.displayItem.type ~= "Flask" then