get rid of event errors when scene not ready

src/plugin/manager.ts

@@ -110,9 +110,8 @@ class Visualization {
                 return;
             }
         });
-
-        this.links.attr('opacity', 1);
-        this.nodes.attr('opacity', 1);
+        this.links?.attr('opacity', 1);
+        this.nodes?.attr('opacity', 1);
 
     }
 
@@ -145,8 +144,8 @@ class Visualization {
             this.fireExternalNullEvent(Config.interactionMouseoutEvent);
         }
 
-        this.links.attr('opacity', 1);
-        this.nodes.attr('opacity', 1);
+        this.links?.attr('opacity', 1);
+        this.nodes?.attr('opacity', 1);
     }
 
     private dragHandler = d3.drag()