[fixes #2701] fix NPE on customized SuperBuilder with Singular

test/transform/resource/before/SuperBuilderSingularCustomized.java

@@ -0,0 +1,9 @@
+import java.util.Set;
+@lombok.experimental.SuperBuilder
+class SuperBuilderSingularCustomized {
+	@lombok.Singular private Set<String> foos;
+    public static abstract class SuperBuilderSingularCustomizedBuilder<C extends SuperBuilderSingularCustomized, B extends SuperBuilderSingularCustomized.SuperBuilderSingularCustomizedBuilder<C, B>> {
+        public B custom(final String value) {
+            return self(); 
+        }
+    }}