Magento 2.3.4 and Elasticsearch: Getting a blank result page when searching

I am having issues with my company’s Magento test site. The issues are when searching using Elastic search, the resulting page is a blank page. Then, when I look for relevant errors in various logs, I see the following PHP error in the /var/log/httpd/error_log file:

[php7:error] [pid 22228] [client xx.xxx.xxx.xxx:xxxxx] PHP Fatal error:  
    Uncaught TypeError: Argument 1 passed to Magento\Elasticsearch\Model\Adapter\FieldMapper\Product\AttributeProvider::getByAttributeCode() 
    must be of the type string, null given, called in /var/www/html/test/vendor/magento/module-elasticsearch/SearchAdapter/Query/Builder/Sort.php 
    on line 92 and defined in /var/www/html/test/vendor/magento/module-elasticsearch/Model/Adapter/FieldMapper/Product/AttributeProvider.php:72nStack 
    trace:n#0 /var/www/html/test/vendor/magento/module-elasticsearch/SearchAdapter/Query/Builder/Sort.php(92): 
    Magento\Elasticsearch\Model\Adapter\FieldMapper\Product\AttributeProvider->getByAttributeCode(NULL)n#1 
    /var/www/html/test/vendor/magento/module-elasticsearch/Elasticsearch5/SearchAdapter/Query/Builder.php(91): 
    Magento\Elasticsearch\SearchAdapter\Query\Builder\Sort->getSort(Object(Magento\Framework\Search\Request))n#2 
    /var/www/html/test/vendor/magento/module-elasticsearch/Elasticsearch5/SearchAdapter/Mapper.php(66): 
    Magento\Elasticsearch\Elasticsearch5\SearchAdapter\Query\Builder->initQue in 
    /var/www/html/test/vendor/magento/module-elasticsearch/Model/Adapter/FieldMapper/Product/AttributeProvider.php 
    on line 72, referer: www.mytestsite.com

Does anyone have any ideas on how to solve this error?

Now, here is my test site’s setup:

PHP version: 7.2.27

Magento version: 2.3.4 community edition

OS: CentOS 7.7.1908

Elasticsearch: 6.8.7-1

Also here is further information on my test site’s setup: I performed an in-place upgrade where I created the test site from a base Magento 2.3.4 install. Then, added all of the modules I needed. Finally, I copied over a backup of my company’s production database and ran the Magento upgrade and compile commands (e.g., “magento setup:upgrade” and “magento setup:di:compile”). And note the production database was based-on Magento 2.3.1.

Thanks in advance for any answer, suggestions, and/or help you can provide.

Regards,

Alvaro