bug fix when annotation contains null article (becuase article id was invalid)

frontend-js/src/main/js/map/data/Annotation.js

@@ -16,7 +16,7 @@ function Annotation(javaObject) {
   } else {
     this.setLink(javaObject.link);
     this.setId(javaObject.id);
-    if (javaObject.article !== undefined) {
+    if (javaObject.article !== undefined && javaObject.article !== null) {
       this.setArticle(new Article(javaObject.article));
     }
     this.setType(javaObject.type);