PluginNotFoundException: The "" plugin does not exist

Actually in case 2, even if the field state appears to be defined in the UI, when you go to the entity add form and try to trigger it, it doesn't work and this warning in thrown:
Warning: Trying to access array offset on value of type null in Drupal\field_states_ui\FieldStateBase->applyState() (line 75 of /var/www/html/web/modules/contrib/field_states_ui/src/FieldStateBase.php)

I looked into this a bit and I can do more on request.

Click the gear icon on admin/structure/types/manage/article/form-display next to the Title field.
In Manage Field States, choose 'Enable' and click 'Add'.

At that point, this code from FieldStateManager::settingsForm will set $type to 'enabled':

        $type = $form_state->getValue([
          'fields',
          $field_name,
          'settings_edit_form',
          'third_party_settings',
          'field_states_ui',
          'form',
          'type',
        ]);

That's because 'third_party_settings' in form_state is like this:

Array
(
    [field_states_ui] => Array
        (
            [form] => Array
                (
                    [type] => enabled
                    [list] => [NULL]
                    [add] => Drupal\Core\StringTranslation\TranslatableMarkup
                )
        )
)

Then, change 'Comparison Type' to 'filled' and it throws a (silent) exception because there's no 'type' key in the 'third_party_settings' array:

Array
(
    [field_states_ui] => Array
        (
            [form] => Array
                (
                    [edit] => Array
                        (
                            [target] => uid
                            [comparison] => filled
                            [value] => [NULL]
                            [submit] => Array
                                (
                                    [save] => Drupal\Core\StringTranslation\TranslatableMarkup
                                    [cancel] => Drupal\Core\StringTranslation\TranslatableMarkup
                                )
                        )
                    [list] => [NULL]
                )
        )
)

The missing 'type' key leads to "The "" plugin does not exist..."

I've been running in to the same error but with "Visible" option.

I'm running into this problem as well for my project, but weirdly enough not on all environments. So I'm nopt sure it I'm experiencing a different error but I do have a boolean field used to toggle other fields.

I get the following error in the log:
TypeError: array_keys(): Argument #1 ($array) must be of type array, null given in array_keys() (line 238 of /web/modules/contrib/field_states_ui/src/FieldStateManager.php)

So it breaks in generating a notice

if (!isset($target['#field_parents'])) {
  $parents = [];
  $this->logger->notice(
    t(
      '#field_parents key not found. This will may cause problems. If so, please report on the @link. For quickest resolution, please include the element details: @details',
      [
        '@link' => Link::fromTextAndUrl(
          t('Field States UI Issue Queue'),
          Url::fromUri('https://www.drupal.org/project/issues/field_states_ui')
        )->toString(),
        '@details' => var_export(array_keys($element[0]),TRUE),
      ]
    )
  )

I mean, I can easily just patch that the var_export isn't used if $element[0] is NULL and yes then the field_state works as expected.
But I'm guessing the real questions is why $element[0] is NULL in this context. I don't have an answer right now since I haven't debugged it (yet).

Any workarounds how we can solve it? I have the same issue with the user's entity.

ended up using the conditional_fields module..

I'm getting this error message regardless at the moment. Whether on a simple text field, boolean, text list, number, etc.

I get same error with current Drupal 10.1.5. Adjusted priority to critical because module is unusable.

Experiencing exactly the same issue. Only if I leave the Target and Comparison fields untouched and then save, does it add the state. I am not able to change either field, or edit an existing state. If I try to edit the existing state, I get:

Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" plugin does not exist. Valid plugin IDs for Drupal\field_states_ui\FieldStateManager are: required, visible, collapsed, unchecked, invisible, optional, checked, enabled, expanded, disabled in Drupal\Core\Plugin\DefaultPluginManager->doGetDefinition() (line 53 of core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php).

I am getting extactly the same issue as alex.verhoeven This issue needs to be resolved.

We are seeing this issue also. We have a new install of the module on Drupal v 10.2.0

Once you choose something in the third-party-settings form for comparison (eg value) or target (other field) - an Ajax Call is made to rebuild the form and the form state with the new plugin type information for the third party settings is gone

Most simply solution for now is just to disable the javascript while configuring the states.
But a patch for this is pretty wanted.

I have bypassed this issue by creating config yaml files manually for the new Field States. If anyone want, I would share my code with them.

@linkanp - care to share your approach? I've been scratching my head for days now.

I'm using the Brave browser, so I just went to the "Shields" Option ( Lion head on address bar), and activated the option "Block Scripts". Then I as able to do the conditions. I'm sure that other browsers have similar options.

I have simplified the issue title because it appears to affect more than just Boolean fields.

I think 2 separate issues are being discussed here.

1. I have created an MR to fix the The "" plugin does not exist bug.
2. I have created a new issue, with an MR, ( 🐛 Unable to change either the Target or Comparison elements Active ) to address the bug with being unable to change either the Target or Comparison elements (first mentioned by @alex.verhoeven, #3371927-12: PluginNotFoundException: The "" plugin does not exist → )