[Bug]: Sveltekit page.subscribe not working in onMount

[alexbjorlig]

Describe the bug

If you subscribe to the page store in onMount, you get the error:

Error: Function called outside component initialization
    at get_current_component (http://localhost:6006/node_modules/.cache/sb-vite/deps/chunk-PLRMHIPC.js?v=3c95bb33:1226:11)
    at getContext (http://localhost:6006/node_modules/.cache/sb-vite/deps/chunk-PLRMHIPC.js?v=3c95bb33:1265:10)
    at Object.subscribe (http://localhost:6006/node_modules/@storybook/sveltekit/src/mocks/app/stores.ts?v=3c95bb33:6:23)
    at http://localhost:6006/src/stories/Button.svelte:110:8
    at run (http://localhost:6006/node_modules/.cache/sb-vite/deps/chunk-PLRMHIPC.js?v=3c95bb33:30:10)
    at Array.map (<anonymous>)
    at http://localhost:6006/node_modules/.cache/sb-vite/deps/chunk-PLRMHIPC.js?v=3c95bb33:2136:50
    at flush (http://localhost:6006/node_modules/.cache/sb-vite/deps/chunk-PLRMHIPC.js?v=3c95bb33:1333:9)
    at init (http://localhost:6006/node_modules/.cache/sb-vite/deps/chunk-PLRMHIPC.js?v=3c95bb33:2217:5)
    at new PreviewRender (http://localhost:6006/node_modules/.cache/sb-vite/deps/chunk-BS7DD6FZ.js?v=3c95bb33:810:5)

To Reproduce

Add this to you .svelte component:

  import { page } from "$app/stores";
  import { onMount } from "svelte";

  onMount(() => {
      page.subscribe(v => console.log('new page!', v))
  })

Or clone my reproduction from here.

System

Storybook Environment Info:

  System:
    OS: macOS 14.3
    CPU: (10) arm64 Apple M1 Max
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.17.1 - ~/.nvm/versions/node/v18.17.1/bin/node
    npm: 10.2.5 - ~/.nvm/versions/node/v18.17.1/bin/npm <----- active
    pnpm: 8.12.0 - ~/.nvm/versions/node/v18.17.1/bin/pnpm
  Browsers:
    Chrome: 121.0.6167.139
    Safari: 17.3
  npmPackages:
    @storybook/addon-essentials: ^7.6.12 => 7.6.12 
    @storybook/addon-interactions: ^7.6.12 => 7.6.12 
    @storybook/addon-links: ^7.6.12 => 7.6.12 
    @storybook/blocks: ^7.6.12 => 7.6.12 
    @storybook/svelte: ^7.6.12 => 7.6.12 
    @storybook/sveltekit: ^7.6.12 => 7.6.12 
    @storybook/test: ^7.6.12 => 7.6.12 
    eslint-plugin-storybook: ^0.6.15 => 0.6.15 
    storybook: ^7.6.12 => 7.6.12

Additional context

Possibly related to this issue.

And sorry for reporting all thesse bugs @JReinhold - but we are keen on getting Storybooks up and running in our Svelte/Sveltekit project 😎