Update tools directive code segment

Author: Thisara Rupasinghe

content/tools/directives/index.md

@@ -36,14 +36,23 @@ public class UppercaseDirective implements SchemaDirectiveWiring {
   @Override
   public GraphQLFieldDefinition onField(SchemaDirectiveWiringEnvironment<GraphQLFieldDefinition> env) {
     GraphQLFieldDefinition field = env.getElement();
-    DataFetcher dataFetcher = DataFetcherFactories.wrapDataFetcher(field.getDataFetcher(), {
-      dataFetchingEnvironment, value ->
-        if (value == null) {
-            return null
-        }
-        return  ((String) value).toUpperCase()
-    })
-    return field.transform({ builder -> builder.dataFetcher(dataFetcher) });
+    GraphQLFieldsContainer parentType = env.getFieldsContainer();
+
+    // build a data fetcher that transforms the given value to uppercase
+    DataFetcher originalFetcher = env.getCodeRegistry().getDataFetcher(parentType, field);
+    DataFetcher dataFetcher =
+        DataFetcherFactories.wrapDataFetcher(
+            originalFetcher,
+            ((dataFetchingEnvironment, value) -> {
+              if (value instanceof String) {
+                return ((String) value).toUpperCase();
+              }
+              return value;
+            }));
+
+    // now change the field definition to use the new uppercase data fetcher
+    env.getCodeRegistry().dataFetcher(parentType, field, dataFetcher);
+    return field;
   }
 }
 ```