Importing RexUI Plugin

[XXXiV-QN]

【Using Phaser 3 & Brackets.io to develop my game】

I am trying to add a slider RexUI plugin to a scene. I added this to my index.html, with the rexuiplugin.min.js I found from another user’s example.
<script type="text/javascript" src="./js/ui/rexuiplugin.min.js"></script>

Then, in my index.js config, I added this:

export var config = {
        plugins: {scene: [{
                key: 'rexUI',
                plugin: rexuiplugin, //load the UI plugins here for all scenes
                mapping: 'rexUI'
            }]
        }
      };

Finally in my slider scene, I tried to use this:
this.rexUI.add.slider({ x: 100, y: 100, ...})

I got this kind of error from console:

My intuition is that I am not adding/loading plugin properly, or has something to do with the rexuiplugin.min.js file? Any suggestion?

I also tried suggestion from this post, didn’t work out for me and was having a similar error.