Replaced alpha 6 code with alpha 7 code on working alpha 6 site. Deleted php and config from files/. ran update.php to see what it would do.
The following code in update.php expects an array.foreach ($config->get('enabled') as $name => $weight) {
The following code in config can return a null.return isset($this->overriddenData[$key]) ? $this->overriddenData[$key] : NULL;
As an experiment, I changed the code to place $config->get('enabled') in a variable then set the variable to array() if null. The change removed the message.