Magento 2 - Error trying Redirect to login page for controller if not logged in

Code below worked for me.

<?php
namespace BACertificateControllerCustAccount;
 
use MagentoCustomerModelSession;
use MagentoFrameworkControllerResultRedirectFactory;
use MagentoFrameworkViewResultPageFactory;

class Index implements MagentoFrameworkAppActionInterface
{
    /**
     * @var MagentoFrameworkViewResultPageFactory
     */
    protected $resultPageFactory;
    /**
     * @var MagentoFrameworkControllerResultRedirectFactory
     */
    protected $resultRedirect;
    /**
     * @var MagentoCustomerModelSession
     */
    protected $customerSession;

    /**
     * @param MagentoFrameworkControllerResultRedirectFactory $resultRedirect
     * @param MagentoFrameworkViewResultPageFactory $resultPageFactory
     * @param MagentoCustomerModelSession $customerSession
     */
    public function __construct(
        RedirectFactory $resultRedirect,
        PageFactory $resultPageFactory,
        Session $customerSession
    ) {
        $this->resultRedirect = $resultRedirect;
        $this->resultPageFactory = $resultPageFactory;
        $this->customerSession = $customerSession;
    }
    /**
     * Default customer account page
     *
     * @return void
     */
    public function execute()
    {
        if (!$this->customerSession->isLoggedIn()) {
            /** @var MagentoFrameworkControllerResultRedirect $resultRedirect */
            $resultRedirect = $this->resultRedirect->create();
            $resultRedirect->setPath('customer/account/login');
            return $resultRedirect;
        } else {
            return $this->resultPageFactory->create();
        }
    }
}

Your error :

PHP Fatal error: Uncaught Error: Call to a member function getRequest() on null in

This lets me to believe that the event does not come with the request attached in it's data array.

Not all observer events have the request attached. You need to check if it is. I suggest setting a breakpoint with xdebug and checking what you have available, that is if the event is not thrown by you, if it is, just add the request in the data array (2nd argument) of the ->dispatch method.

$this->_eventManager->dispatch(
    'controller_action_predispatch',
    [ 'request' => $this->getRequest() ] //if request is available...
);

If it's not thrown by you and you need it in your observer just inject the RequestInterface in the observer and you'll get the current request.

<?php
class MyObserver implements ObserverInterface
{
        const VALID_ACTION = 'full_action_name';

    /**
    * @var MagentoFrameworkAppRequestInterface
    */
    private $request;

    /**
    * MyObserver constructor.
    *
    * @param MagentoFrameworkAppRequestInterface $request
    */
    public function __construct(
        RequestInterface $request
    ) {
        $this->request = $request;
    }

    /**
    * @param Observer $observer
    *
    * @return void
    */
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        if ($this->request->getFullActionName() === self::VALID_ACTION ) {
            //Do something here... 
        }
    }
}

Note that full action name is the module_controller_action not just the action name as you have them in your array, so it will not match. You just need to get the action name to have them match.

try this

 /**
     * @var MagentoFrameworkAppResponseInterface
     */
    protected $_response;

    /**
     * @var MagentoStoreModelStoreManagerInterface
     */
    protected $_storeManager;

    public function __construct
    (
        MagentoFrameworkUrlInterface $url,
        MagentoStoreModelStoreManagerInterface $storeManagerInterface,
        MagentoFrameworkAppResponseInterface $response 
    ){
        $this->_storeManager = $storeManagerInterface;
        $this->_response = $response; 
    }



    public function execute(MagentoFrameworkEventObserver $observer)
    {
        $storeObj = $this->_storeManager->getStore(1);
        $BaseURL = $storeObj->getBaseUrl(MagentoFrameworkUrlInterface::URL_TYPE_WEB);
        $url = $BaseURL . 'customer/account/login';
        $this->_response->setRedirect($url)->sendResponse();

    }

protected $_url; 
public function __construct(
        MagentoFrameworkAppActionContext $context,
        MagentoFrameworkUrlInterface $url
    )
    {
        parent::__construct($context);
        $this->_url = $url;

    }

public function execute()
    {

......
.....
.....
$redirectUrl= $this->_url->getUrl('customer/account/login');
        $this->_responseFactory->create()->setRedirect($redirectUrl)->sendResponse();

}

This is working for me.