Custom field plugin: Unable to find template / General custom field plugin information
Where are you storing your mapElement template? - Unless it is stored in the same folder as the templates for the front end of your site, then 

Craft::$app-&gt;getView()-&gt;setTemplateMode(View::TEMPLATE_MODE_SITE);


is telling craft to look in the wrong place.

If you've got it stored in your plugin directory, then you don't need to change the template mode at all, as per : https://docs.craftcms.com/v3/extend/updating-plugins.html#rendering-templates