effects.js causes Fatal Error

[tk-90-github]

Describe the bug

Push to 'null' causes total crash of project.

The error causing line is at src/internal/client/reactivity/effect.js:270
context.l.r1.push(token);

Error message:

Uncaught (in promise) TypeError: Cannot read properties of null (reading 'r1')

Reproduction

In a seperate project, which I created by the template doesn't show that effect.
Sorry, I'm not so deep into svelte-internals to know how this bug can be reproduced

External libs that caused this error:
"svelte-fa"
"svelte-file-dropzone"

I tried to add some icons with Fa in the basic example but it doesn't show the effect I face in production.

Logs

Uncaught (in promise) TypeError: Cannot read properties of null (reading 'r1')

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (20) x64 12th Gen Intel(R) Core(TM) i9-12900HK
    Memory: 6.86 GB / 31.68 GB
  Binaries:
    Node: 22.11.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.22 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 9.6.4 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.7.0 - ~\scoop\shims\pnpm.EXE
  Browsers:
    Edge: Chromium (130.0.2849.46)
    Internet Explorer: 11.0.22621.3527
  npmPackages:
    svelte: ^5.1.9 => 5.1.16
    webpack: ^5.96.1 => 5.96.1

Severity

blocking all usage of svelte

[tk-90-github]

I manually edit the node modules in my project and came up with following solution:

/**
 * Internal representation of `$: ..`
 * @param {() => any} deps
 * @param {() => void | (() => void)} fn
 */
export function legacy_pre_effect(deps, fn) {
	var context = /** @type {ComponentContextLegacy} */ (component_context);

	/** @type {{ effect: null | Effect, ran: boolean }} */
	var token = { effect: null, ran: false };

	if(context.l !=null &&context.l.r1 !=null ){
		context.l.r1.push(token);
	}

	token.effect = render_effect(() => {
		deps();

		// If this legacy pre effect has already run before the end of the reset, then
		// bail out to emulate the same behavior.
		if (token.ran) return;

		token.ran = true;

		if(context.l!=null && context.l.r2 !=null){
		      set(context.l.r2, true);
		}
		untrack(fn);
	});
}

Edit:
Watch out for the line set(context.l.r2, true); which was unprotected, too.

[dummdidumm]

Please provide a reproduction repository. You can use stackblitz or push a github repository.

[anna-is-cute]

This is a hard issue to provide a reproduction for. I can only reproduce this in production (dev servers do not produce the error nor does vite preview), it's only a client-side error (SSR works fine and then hydration breaks it), and it happens for all legacy-specific code. That last bit makes sense, given that the legacy-specific field in the context is null.

Best I can do is a private staging environment with source maps. If someone on the Svelte team wants to investigate, I can give them access. Drop me a line at niece-clavicle-gem (at) duck (dot) com.