EntityMalformedException with select og_audience widget if group content has no other fields

I succeeded to reproduce the error.

1. Install a minimal Drupal.
2. Enable og_ui (with all its dependencies) and field_ui.
3. Create a content type "Group" and select "Group" on the "Organic Group" tab on the content type edit page.
4. Add a Group node.
5. Create a content type "Page" and select "Group content" on the "Organic Group" tab on the content type edit page. Select 'Group' as bundle.
6. Remove the body field from Page so that the only remaining field is og_group_ref. The cardinality and requiredness of og_group_ref does not seem to matter.
6a. EDIT: Change the og_group_ref widget from OG Reference to Select.
7. Try to create a page by submitting the node/add/page form.

You should see a white error page with the following text:

Notice: Undefined index: field_mode in OgBehaviorHandler->validate() (line 246 of xxx/sites/all/modules/og/plugins/entityreference/behavior/OgBehaviorHandler.class.php).
Notice: Undefined index: #entity in og_form_group_reference_validate() (line 754 of xxx/sites/all/modules/og/og.module).
EntityMalformedException: Missing bundle property on entity of type node. in entity_extract_ids() (line 7697 of xxx/includes/common.inc).

The problem comes from the core field_default_form() which creates $form['#entity'] only

  if (!isset($get_delta) && field_behaviors_widget('multiple values', $instance) == FIELD_BEHAVIOR_DEFAULT)

However, $form['#node'] is always added by node_form().

EDIT: I left out one thing: The widget of the og_group_ref field must be 'Select', not the default OG Reference. It is not clear yet, whether selects should be allowed at all for og_group_ref fields. See #2141101: Plain core select widget does not work for og_group_ref fields.

Why is this a duplicate? This is a different issue with a different patch.

Yes, it does exist. It is related to #2141101: Plain core select widget does not work for og_group_ref fields since it only happens when using the select widget but it is a different issue. Do the steps to reproduce in #2 not work for you?

I think you're right. There are two problems in og_form_group_reference_validate():

1. $form_state[$entity_type] is not necessarily set and therefore it thinks "We are inside field settings page". (For example in the case of taxonomy_terms instead of $form_state['taxonomy_term'] $form_state['term'] is set.) We need a different logic here to determine where we are.
2. Even if it did not think that we are inside a field settings page $form['#entity'] is not necessarily set. Whether it is set or not is decided in field_default_form(). We should check first if $form['#entity'] is set and if not, we should try and get $entity from $form['og_group_ref'][LANGUAGE_NONE][0]['#entity'] (or whatever $field_name og_get_group_audience_fields() is returning.)

What do you think?

The patch does the following:

1. og_form_group_reference_validate() does not get called when submitting a field settings page. My guess is that 2 years ago when the 4 lines in #11 got commited it did get called but the validation has got refactored into OgBegaviorHandler::validate() since. So the patch removes these 4 lines.
2. It tries to get the entity from $form['#entity'] and if it fails it gets it from the og_audience field form element's #entity property as that's where it should be according to field_default_form().

13: og-entity_key-2143733-13.patch queued for re-testing.

Yes, it is the scenario described in comment #2. If (as most often is the case) $form['#entity'] is set this issue does not appear.