Tree: Remove undefined variable, fixed #1945

packages/tree/src/tree-node.vue

@@ -133,12 +133,12 @@
         this.tree.$emit('current-change', store.currentNode ? store.currentNode.data : null, store.currentNode);
         this.tree.currentNode = this;
         if (this.tree.expandOnClickNode) {
-          this.handleExpandIconClick(event);
+          this.handleExpandIconClick();
         }
         this.tree.$emit('node-click', this.node.data, this.node, this);
       },
 
-      handleExpandIconClick(event) {
+      handleExpandIconClick() {
         if (this.expanded) {
           this.node.collapse();
         } else {