Fix crash when rejoining as Vix.

mods/artifact_characters/init.lua

@@ -34,7 +34,9 @@ function ns.apply_vix(m)
     m.eye_height = 1.5
     -- Switch hand appearance.
     m.inv:set_stack("main", 1, ItemStack("input_"..m.character))
+    if m.healthbar then
         m.object:hud_change(m.healthbar, "text", "artifact_heart_vix.png")
+    end
     if m.blackrod then
         m.blackrod:remove()
         m.blackrod = nil