Fix NPE in a validator from registry access refactor

src/main/java/carpet/CarpetSettings.java

@@ -977,6 +977,7 @@ public class CarpetSettings
 
         @Override
         public String validate(CommandSourceStack source, CarpetRule<String> currentRule, String newValue, String string) {
+            if (source == null) return newValue; // closing or sync
             Optional<Block> ignoredBlock = source.registryAccess().registryOrThrow(Registries.BLOCK).getOptional(ResourceLocation.tryParse(newValue));
             if (!ignoredBlock.isPresent()) {
                 Messenger.m(source, "r Unknown block '" + newValue + "'.");