Errors on auto-restarted local preview (SvelteKit 3)

[stephenlrandall]

Describe the bug

When restarting the dev server while a browser tab with the local preview is already open, the tab will reload. Often -- but not always -- this leads to an error that doesn't go away until the tab is refreshed.

reload-issue-kit-3.mov

Reproduction

1. Create a SvelteKit 3 project (sample repo: https://github.com/stephenlrandall/kit-3-reload-bug, which is just the Sverdle demo app, with npx sv@next migrate sveltekit-3 --tasks all --confirm applied)
2. Start the dev server, then open in a browser
3. Kill the dev server, but leave the tab open
4. Restart the dev server, and let the tab auto-reload

Logs

randasl1@JC7DV02104-ML test-app % pnpm dev
$ vite dev
2:15:34 PM [vite] (ssr) connected.

  VITE v8.2.1  ready in 286 ms

  ➜  Local:   http://localhost:5173/
  ➜  Network: use --host to expose
  ➜  press h + enter to show help
2:15:35 PM [vite] (ssr) page reload .svelte-kit/generated/shared/error-template.js
2:15:35 PM [vite] (ssr) program reload
2:15:35 PM [vite] (ssr) page reload .svelte-kit/generated/server/internal.js
2:15:35 PM [vite] (ssr) program reload
TypeError: Cannot read properties of null (reading 'function')
    at Module.push_element (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/dev.js:59:25)
    at eval (/Users/randasl1/Desktop/test-app/src/routes/+layout.svelte:6:16)
    at Renderer.child (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:214:18)
    at Renderer.component (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:319:22)
    at _layout (/Users/randasl1/Desktop/test-app/src/routes/+layout.svelte:3:2)
    at eval (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/root.svelte:22:18)
    at Renderer.boundary (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:266:19)
    at node (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/root.svelte:38:17)
    at eval (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/root.svelte:43:10)
    at Renderer.child (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:214:18)
TypeError: Cannot read properties of null (reading 'function')
    at Module.push_element (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/dev.js:59:25)
    at eval (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/error.svelte:3:1)
    at Renderer.child (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:214:18)
    at Renderer.component (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:319:22)
    at Error (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/error.svelte:4:2)
    at failed (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/root.svelte:23:14)
    at Renderer.boundary (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:304:5)
    at node (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/root.svelte:38:17)
    at eval (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/root.svelte:43:10)
    at Renderer.child (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:214:18)
TypeError: Cannot read properties of null (reading 'function')
    at Module.push_element (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/dev.js:59:25)
    at eval (/Users/randasl1/Desktop/test-app/src/routes/+layout.svelte:6:16)
    at Renderer.child (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:214:18)
    at Renderer.component (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:319:22)
    at _layout (/Users/randasl1/Desktop/test-app/src/routes/+layout.svelte:3:2)
    at eval (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/root.svelte:22:18)
    at Renderer.boundary (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:266:19)
    at node (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/root.svelte:38:17)
    at eval (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/@sveltejs+kit@3.0.0-next.23_@sveltejs+vite-plugin-svelte@7.3.0_svelte@5.56.9_vite@8.2.1_4295e3b78eb1277a247c4069ea5d8747/node_modules/@sveltejs/kit/src/runtime/components/root.svelte:43:10)
    at Renderer.child (/Users/randasl1/Desktop/test-app/node_modules/.pnpm/svelte@5.56.9/node_modules/svelte/src/internal/server/renderer.js:214:18)
500 GET /
200 GET /_app/env.js

System Info

System:
    OS: macOS 26.6.1
    CPU: (14) arm64 Apple M4 Pro
    Memory: 1.91 GB / 48.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 26.2.0 - /Users/randasl1/.nvm/versions/node/v26.2.0/bin/node
    npm: 11.13.0 - /Users/randasl1/.nvm/versions/node/v26.2.0/bin/npm
    pnpm: 11.22.0 - /Users/randasl1/Library/pnpm/bin/pnpm
  Browsers:
    Chrome: 151.0.7922.138
    Edge: 151.0.4129.86
    Safari: 26.6
  npmPackages:
    @sveltejs/adapter-auto: next => 8.0.0-next.3 
    @sveltejs/kit: next => 3.0.0-next.23 
    @sveltejs/vite-plugin-svelte: ^7.1.2 => 7.3.0 
    svelte: ^5.56.4 => 5.56.9 
    vite: ^8.0.16 => 8.2.1

Severity

annoyance

Additional Information

In other projects, the error is different. I've also seen complaints about using page.data inappropriately and using invalid snippet arguments.

[teemingc]

This one's really tricky to reproduce. I got the same error once (along with some Vite "outside allowed serving list" messages) but never again after recreating and reinstalling the projects multiple times.

[stephenlrandall]

@teemingc If you have any ideas about what might be causing it, I'm happy to help debug since the issue happens fairly regularly for me. I just have no idea where to start, since all I can see is that it seems to be caused by ssr_context not being set by the time push_element is called. But I have no context for where those things are used/called.

[teemingc]

@stephenlrandall if you can create repo so I can pin down the specific versions causing this it'll help a lot. You can also try updating to the latest versions and check if the error still occurs

[stephenlrandall]

@teemingc Added a sample repo to the issue description. Also re-installed my dependencies, but still seeing the issue.