fix: handle undefined this.el in findEle function

Author: SSShooter

package.json

@@ -1,6 +1,6 @@
 {
   "name": "mind-elixir",
-  "version": "5.0.0",
+  "version": "5.0.1",
   "type": "module",
   "description": "Mind elixir is a free open source mind map core.",
   "keywords": [

src/utils/dom.ts

@@ -7,7 +7,7 @@ import { layoutChildren } from './layout'
 // DOM manipulation
 const $d = document
 export const findEle = function (this: MindElixirInstance, id: string, el?: HTMLElement) {
-  const scope = this.el ? this.el : el ? el : document
+  const scope = this?.el ? this.el : el ? el : document
   const ele = scope.querySelector<Topic>(`[data-nodeid=me${id}]`)
   if (!ele) throw new Error(`FindEle: Node ${id} not found, maybe it's collapsed.`)
   return ele