I'm getting a server error when adding extensions to checkout

[fullmetal]

Every time I try to add an extension from an app (my custom extension or other 3rd party apps) I'm getting a Server error. 

The only error I'm seeing in console is this:

Uncaught DOMException: Failed to read the 'localStorage' property from 'Window': Access is denied for this document.
    at i (https://cdn.shopify.com/shopifycloud/checkout-web/assets/app.latest.en.4edff2cf02c85495b44e.js:1:934622)
    at a (https://cdn.shopify.com/shopifycloud/checkout-web/assets/app.latest.en.4edff2cf02c85495b44e.js:1:934135)
    at https://cdn.shopify.com/shopifycloud/checkout-web/assets/app.latest.en.4edff2cf02c85495b44e.js:1:625984
    at 42849 (https://cdn.shopify.com/shopifycloud/checkout-web/assets/app.latest.en.4edff2cf02c85495b44e.js:1:626159)
    at n (https://cdn.shopify.com/shopifycloud/checkout-web/assets/runtime.latest.en.f1c70d3f9ed6a84ebed0.js:1:151)
    at https://cdn.shopify.com/shopifycloud/checkout-web/assets/app.latest.en.4edff2cf02c85495b44e.js:1:1098829
    at n.O (https://cdn.shopify.com/shopifycloud/checkout-web/assets/runtime.latest.en.f1c70d3f9ed6a84ebed0.js:1:433)
    at https://cdn.shopify.com/shopifycloud/checkout-web/assets/app.latest.en.4edff2cf02c85495b44e.js:1:1098847
    at t (https://cdn.shopify.com/shopifycloud/checkout-web/assets/runtime.latest.en.f1c70d3f9ed6a84ebed0.js:1:6779)
    at https://cdn.shopify.com/shopifycloud/checkout-web/assets/app.latest.en.4edff2cf02c85495b44e.js:1:67

I'm not sure if this has something to do with the useStorage function but otherwise it doesn't seem to apply to this app