Error in Svelte runtime error handling but lacks minimal repro.
The issue involves a runtime error in Svelte's error handling mechanism when accessing properties of null, specifically in the `handle_error` method. The problem requires understanding Svelte's internal update cycle and error handling, but lacks a minimal reproduction case, making it harder to diagnose and fix.
This error started popping up after we upgraded Svelte from v5.47.1 to v5.48.0, and it is still present in the latest version v5.49.2
Here's the error logged in the console:
Uncaught TypeError: Cannot read properties of null (reading 'f')
at handle_error (error-handling.js:20:27)
at update_reaction (runtime.js:342:10)
at update_effect (runtime.js:454:18)
at flush_queued_effects (batch.js:696:4)
at _Batch.process (batch.js:227:4)
at flush_effects (batch.js:640:10)
at _Batch.flush (batch.js:355:4)
at Array.<anonymous> (batch.js:528:12)
at run_all (utils.js:46:8)
at run_micro_tasks (task.js:10:2)
And here's a public link to one of our Sentry alerts, with a more thorough stack trace of the code that caused the error: https://kvist.sentry.io/share/issue/ce1baf62f21c4928bdf9331ceae87721/
I still haven't managed to create a minimal repro, as i am not that familiar with the internal setup of Svelte's update-cycle, so creating a minimal reproduction would take quite some time. I hope that the information is sufficient enough, and if it isn't, then let me know and i'll try my best to help.
It seems to me like error is thrown because of this (active_reaction).f accessor in the handle_error method
// for unowned deriveds, don't throw until we read the value
if (effect === null) {
/** @type {Derived} */ (active_reaction).f |= ERROR_VALUE;
return error;
}
System:
OS: macOS 26.2
CPU: (10) arm64 Apple M2 Pro
Memory: 4.90 GB / 32.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 24.13.0 - /Users/mathiaspicker/.local/share/mise/installs/node/24.13.0/bin/node
npm: 11.6.2 - /Users/mathiaspicker/.local/share/mise/installs/node/24.13.0/bin/npm
pnpm: 10.28.2 - /opt/homebrew/bin/pnpm
bun: 1.2.22 - /opt/homebrew/bin/bun
Browsers:
Brave Browser: 143.1.85.117
Chrome: 144.0.7559.112
Edge: 144
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