Fix link and group without labels

EditorGroup.js

@@ -32,7 +32,7 @@ class EditorGroup extends EditorEntryBase {
     return super.serialize({
       type: 'group',
       label: this.getLabel(),
-      labelObj: this.getLabelObj().serialize(),
+      labelObj: this.getLabelObj() ? this.getLabelObj().serialize() : null,
       members: this.nodes_.serialize(EditorNode),
     });
   }

EditorLink.js

@@ -30,7 +30,7 @@ class EditorLink extends EditorEntryBase {
     return super.serialize({
       type: 'link',
       label: this.getLabel(),
-      labelObj: this.getLabelObj().serialize(),
+      labelObj: this.getLabelObj() ? this.getLabelObj().serialize() : null,
       from: this.getFrom().serialize(),
       to: this.getTo().serialize(),
     });

GraphGroup.js

@@ -28,7 +28,7 @@ class GraphGroup {
     let group = new GraphGroup();
     group.id = item.id;
     group.label = item.label;
-    group.labelId = item.labelObj.id;
+    group.labelId = item.labelObj ? item.labelObj.id : null;
     group.nodeLabels = new Set();
     for (let member of item.members) {
       if (member.label == '') {

GraphLink.js

@@ -33,7 +33,7 @@ class GraphLink {
     let link = new GraphLink();
     link.id = item.id;
     link.label = item.label;
-    link.labelId = item.labelObj.id;
+    link.labelId = item.labelObj ? item.labelObj.id : null;
     link.fromLabel = item.from.label;
     link.toLabel = item.to.label;
     if (link.fromLabel == '' || link.toLabel == '') {