Error from i18n : Notice: Undefined index: form_id in i18n_field_field_widget_form_alter()

That's not an error, but a 'notice', just disable them for your live site.
No idea why, are you using latest stable versions from everything else? (Drupal core, etc..)

The module is suppossed to work with latest Drupal core version, and we cannot test with older versions / combinations.

Anyway, just disable notices for now till you can upgrade. Or try the last i18n version released before Drupal 7.15

Check out:
https://drupal.org/node/3060/release?api_version[]=103
https://drupal.org/node/5917/release?api_version[]=103

FYI, the full text of the notice is:

"Notice : Undefined index: form_id in i18n_field_field_widget_form_alter() 
(line 179 in ...\modules\i18n\i18n_field\i18n_field.module)"

- The initial i18n installation was made on a Drupal 7.12 config with the version 7.x-1.8+2-dev,
- I came across the field translation issue : Labels are not translated with i18n_field for Drupal core,
- So I upgraded i18n to the 7.x-1.9+6-dev version and the notice popped up.
- There is one notice per each field that was not translated correctly with the original i18n version,
- I tried to upgrade the Variable module as well to the version 7.x-2.3 but it changed nothing,
- Same symptoms with the i18n stable version 7.x-1.10.

Thanks a lot for your much appreciated work, Jose.

I have the same error. i don't know why but in some case i dont have 'build_info' key defined in the form_state array.
The i18n_field_field_widget_form_alter() function is checking the form_id without checking if the ket $form_state['build_info']['form_id'] is defined and that causes the notice.

i have added a patch with a simple isset() test

this one is working

I can confirm that this problem still exists in certain circumstances. The fix at #8 solved the issue for me as well.

Issue happening in a drupal 7.43 site and with last i18n versions. Fix at #8 also worked for me.

#8 worked for me too

After the i18n 7x-1.18 release i don't need the patch anymore.
https://www.drupal.org/project/i18n/releases/7.x-1.18

Does this patch solved this issue?
https://www.drupal.org/node/2386515

I can confirm that this problem still exists in certain circumstances. The fix at #8 solved the issue for me as well.

My Environment:

Drupal: 7.78
i18n: 7.x-1.27
Database 4 byte UTF-8 support: Enabled
PHP: 7.4.16
Web-server: Apache/2.4.46 (Ubuntu)
DBMS version: 8.0.23

Thanks