The cause of this error is wrong return type of your before
plugin: you must return result as array.
For example:
namespace Df\Framework\App;
use \Magento\Framework\App\ActionInterface;
use \Magento\Framework\App\RequestInterface;
class ActionInterfacePlugin {
/**
* @param ActionInterface $subject
* @param RequestInterface $request
* @return RequestInterface
*/
public function beforeDispatch(ActionInterface $subject, RequestInterface $request) {
rm_state()->actionSet($subject);
return [$request];
}
}