Cannot save configuration with developer mode enabled

When Magento developer mode is enabled, attempting to save the system config results in the message:
> An error occurred while saving this configuration: Notice: Trying to get property of non-object in /path/to/magento/app/code/core/Mage/Adminhtml/Model/Config/Data.php on line 135

The "trying to get property of non-object" error is allowed when not in the strict mode enabled by Magento developer mode.

Problem is caused because the custom fields are added dynamically only to the adminhtml form block, not the config object which is used for validating and persisting the configuration being saved.

Pull request attached.