Partially fix #6880 (#6888)

patches/server/Use-correct-LevelStem-registry-when-loading-default-.patch

@@ -15,8 +15,10 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
              List<CustomSpawner> list = ImmutableList.of(new PhantomSpawner(), new PatrolSpawner(), new CatSpawner(), new VillageSiege(), new WanderingTraderSpawner(iworlddataserver));
 -            LevelStem worlddimension = (LevelStem) registrymaterials.get(dimensionKey);
 +            // Paper start - Use correct LevelStem registry
-+            LevelStem worlddimension = generatorsettings.dimensions().get(dimensionKey);
-+            if (worlddimension == null) {
++            final LevelStem worlddimension;
++            if (dimensionKey == LevelStem.END || dimensionKey == LevelStem.NETHER) {
++                worlddimension = generatorsettings.dimensions().get(dimensionKey);
++            } else {
 +                worlddimension = registrymaterials.get(dimensionKey);
 +            }
 +            // Paper end