修改

public/index.html

@@ -22,6 +22,7 @@
   <div id="app"></div>
   <!-- built files will be auto injected -->
   <script src="https://unpkg.com/bpmn-js@6.0.2/dist/bpmn-viewer.development.js"></script>
+  <script src="./leader-line.min.js"></script>
 </body>
 
 </html>