Steps to reproduce the issue

Attempt to submit a custom component form and save a record.

Expected result

Record saves and page redirects to empty form

Actual result

Returns HTTP 500

System information (as much as possible)

Joomla! 3.7 upgraded from 3.6.5
PHP 7.1.4

Additional comments

Found that the error happens when my custom component calls $model->getForm(). After finding that disabling the System - Fields plugin allowed my form to work, I traced the issue through:

JModel::loadForm()
JModel::preprocessForm()
PlgSystemFields::onContentPrepareForm()
FieldsHelper::extract()

and found that it is the call to class_exists() on Line 55 of administrator/components/com_fields/helpers/fields.php that was causing the error.

if (class_exists($cName) && is_callable(array($cName, 'validateSection')))
{

$section = call_user_func_array(array($cName, 'validateSection'), array($parts[1], $item));

if ($section)
{
$parts[1] = $section;
}
}

I'm not entirely sure what this code is supposed to do as I can find no documentation explaining the purpose.

I found two choices to make my custom component work again... leave "System - Fields" disabled, or add the second parameter to the class_exists() call to not attempt to autoload the non-existent class.

"if (class_exists($cName, false) && is_callable(array($cName, 'validateSection')))"

I chose the latter for now. Hopefully someone can shed some light and provided clearer direction.

Thanks.