TypeError: Cannot read property 'add' of undefined

[jusjus112]

• Your OS
  Linux Ubuntu - Webstorm 2021.1.3

• Node.js version
  v12.13.1.

• Vuesax version
  4.0.1-alpha.25

• Browser version
  FireFox Developer Edition 91.0b3

• Which package manager did you use for the install?
  Used nuxt-create-app
  Using latest NuxtJS to create an app with.

• What did you do when it happened?
  I clicked the button and after it disappeared it thrown the error.

Created a notification with a vs-button, see code here: https://hastebin.com/emequkunam.xml.

• Steps to reproduce

1. Create buttons with the code on the link at hastebin. (I used NuxtJS)
2. Click the black button, a notification will pop up.
3. After it is fading out, the error is thrown and the application gives an error.

• Full Error:

TypeError: can't access property "add", el.classList is undefined
    setColor vuesax.js:18277
    handleIsVisible vuesax.js:25842
    VueJS 5
    handleIsVisible vuesax.js:25841
    VueJS 12
    close vuesax.js:25792
    notification vuesax.js:26065
client.js:102
    _callee$ NuxtJS
    Babel 8
    VueJS 7
    handleIsVisible vuesax.js:25841
    VueJS 12
    close vuesax.js:25792
    notification vuesax.js:26065```

- Dependencies

"dependencies": {
"@nuxt/content": "^1.14.0",
"@nuxtjs/axios": "^5.13.6",
"bootstrap": "^5.0.2",
"bootstrap-vue": "^2.21.2",
"boxicons": "^2.0.7",
"core-js": "^3.15.1",
"express": "^4.17.1",
"helmet": "^4.6.0",
"nuxt": "^2.15.7",
"ol": "^5.3.3",
"vue": "^2.6.14",
"vuelayers": "^0.11.36",
"vuesax": "^4.0.1-alpha.25"
},```