Try this:

public function onContentPrepareForm($form, $data) {
   if (isset($data) && count($data)) {
      // get article params
      $attribs = $data->get('attribs');
   }
}

this is not correct because I'm still in the article and I need to edit the attribs

I have used this to avoid the error, but the problem is in the fact that $data shall be a JObject and not an Array

if (method_exists($data,'get')) {
$attribs = $data->get('attribs');
} else if (isset($data['attribs'])) {
$attribs = $data['attribs'];
}

That was my misunderstanding, yes you are right $data should remain JObject

You need to figure out what type of data you get, take a look at this: https://github.com/joomla/joomla-cms/blob/staging/plugins/system/fields/fields.php#L194

I'm already checking the context, I'm in 'com_content.article' in both cases

this problem was not there before the 3.7

Check it with 3.6.5 it is the same you get an empty array or a jObject

I'm having an similair issue, but without a plugin as above.
Simply when creating an article and then change the category.
Please see: https://forum.joomla.org/viewtopic.php?f=706&t=950583
I have no further info then the Error itself.

@rdeutz : no matter in which version this has been introduced, it just does not work as expected :)

oh no, it works as expected, if you check the profile plugin you will find:

/**
    * Adds additional fields to the user editing form
    *
    * @param   JForm  $form  The form to be altered.
    * @param   mixed  $data  The associated data for the form.
    *
    * @return  boolean
    *
    * @since   1.6
    */

$data has a type of mixed. No question that this is not a brillant idea to have different types for a parameter but thats's how it is.

Closing not a bug

ok I agree. Then we don't investigate more to change this ?
We just close it ... it's a shame

You are free to make a pull request or open a ticket with a suggestion how to change it, this ticket gives the impression we have to fix a bug.

ok Robert I understand :)
Then it is not a bug, but something we could improve (my 2 cents).

I will then investigate.
Thanks !