Use get() to get data from DataSet/View instead of directly accessing member _data. (#3069)

lib/network/modules/NodesHandler.js

@@ -330,7 +330,7 @@ class NodesHandler {
       if (nodes.hasOwnProperty(nodeId)) {
         node = nodes[nodeId];
       }
-      let data = this.body.data.nodes._data[nodeId];
+      let data = this.body.data.nodes.get(nodeId);
       if (node !== undefined && data !== undefined) {
         if (clearPositions === true) {
           node.setOptions({x:null, y:null});