Use else branch to report second error

Author: benjie

src/validation/rules/SingleFieldSubscriptionsRule.js

@@ -26,6 +26,20 @@ export function SingleFieldSubscriptionsRule(
               node.selectionSet.selections.slice(1),
             ),
           );
+        } else {
+          const selection = node.selectionSet.selections[0];
+          const fieldName = selection.name.value;
+          // fieldName represents an introspection field if it starts with `__`
+          if (fieldName[0] === '_' && fieldName[1] === '_') {
+            context.reportError(
+              new GraphQLError(
+                node.name
+                  ? `Subscription "${node.name.value}" must not select an introspection top level field.`
+                  : 'Anonymous Subscription must not select an introspection top level field.',
+                node.selectionSet.selections,
+              ),
+            );
+          }
         }
         const selection = node.selectionSet.selections[0];
         const fieldName = selection.name.value;