I'm just testing the module for a different error and came across this one:
TypeError: node_view(): Argument #1 ($node) must be of type Node, User given, called in /app/httpdocs/modules/leaflet/modules/leaflet_views/leaflet_views_plugin_style.inc on line 322 in node_view() (line 1115 of /app/httpdocs/core/modules/node/node.module).
It might be as simple as changing the line to be:
$build = entity_view($entity, $this->options['view_mode']);
I got this error after adding a geofield to the user account, and then adding a view to display the location and choosing <user entity> for displaying the description.
The popup appears empty so there might be a different problem as well.