ScrollMagic + GSAP gives me “window is not defined” on "gridsome build"

After installing both and combining them to work together, ScrollMagic gives me a "window is not defined" error. I've already tried many solutions, but I'm still struggling with it. Can someone help me, please? Or suggest a good alternative for ScrollMagic+GSAP?

([Link](https://github.com/OziOcb/pozyzniewski-portfolio/tree/scrollmagic-issue) to the repo on GitHub.)

I installed them via yarn.
`yarn add scrollmagic gsap`

In main.js I added:
(I followed [this](https://github.com/pirony/ks-vue-scrollmagic/issues/13))
```javascript
/* GSAP & ScrollMagic */
import ScrollMagic from "scrollmagic"
import "imports-loader?define=>false!scrollmagic/scrollmagic/uncompressed/plugins/animation.gsap"

// Load ScrollMagic's Indicators only on development environment
if (process.env.NODE_ENV === "development") {
require("imports-loader?define=>true!scrollmagic/scrollmagic/uncompressed/plugins/debug.addIndicators")
}

const GSAPScrollMagic = {
install(Vue, options) {
// GSAP
Vue.prototype.$GSAP = {
TweenMax,
TimelineMax,
Linear,
Power1,
Power2,
Power3,
Back
}

// ScrollMagic
Vue.prototype.$ScrollMagic = {
Controller: new ScrollMagic.Controller(),
Scene: ScrollMagic.Scene
}
}
}

export default function(Vue, { router, head, isClient }) {
// Add GSAP & ScrollMagic to the project
Vue.use(GSAPScrollMagic)
}
```

And in my pages/index.vue file i simply add it like this:
```javascript

export default {
...
mounted() {
// create a Tween
const testTween = new this.$GSAP.TimelineMax()
testTween
.from(".smElement", 1.5, { ease: this.$GSAP.Power2.easeOut, opacity: 0 })

// create a scene
const testScene = new this.$ScrollMagic.Scene({
triggerElement: ".smElement",
triggerHook: 0.7,
duration: 300
})
.addIndicators()
.setTween(testTween)

// Add the scene to the controller
this.$ScrollMagic.Controller.addScene(testScene)
}
}

```

everything works great on 'gridsome develop'.
On 'gridsome build' I get this error:
```bash
Gridsome v0.7.10

Initializing plugins...
Load sources - 0.06s
Create GraphQL schema - 0.09s
Create pages and templates - 0.05s
Generate temporary code - 0.06s
Bootstrap finish - 1.92s
10% building 1/1 modules 0 active
Update available: 0.3.1
Run yarn add global @gridsome/cli to update

Compile assets - 20.38s
Execute GraphQL (6 queries) - 0.02s
Write out page data (6 files) - 0s
Could not generate HTML for "/blog/test/":
ReferenceError: window is not defined
at Object. (node_modules/scrollmagic/scrollmagic/uncompressed/ScrollMagic.js:37:0)
at assets/js/app.a4f12461.js:3124:37
at Object. (node_modules/scrollmagic/scrollmagic/uncompressed/ScrollMagic.js:27:1)
at __webpack_require__ (webpack/bootstrap:25:0)
at Module. (assets/js/app.a4f12461.js:12578:19)
at __webpack_require__ (webpack/bootstrap:25:0)
at Object. (assets/js/app.a4f12461.js:8588:18)
at __webpack_require__ (webpack/bootstrap:25:0)
at assets/js/app.a4f12461.js:118:18
at Object. (assets/js/app.a4f12461.js:121:10)
```
#
I've found [a similar issue with ScrollMagic on Nuxt.js](https://stackoverflow.com/questions/47844836/nuxtjs-with-scrollmagic-gives-me-window-is-not-defined), but I'm not sure how to convert the solution to Gridsome.

I've also tried to follow [the official docs for adding external script without SSR support](https://gridsome.org/docs/assets-scripts/#without-ssr-support), with no luck...