The Vaddin version of the dashboard shows negative availability sometimes

I just discovered this problem on 18.0.1 and a branch of develop updated today.

My test system is a VM with CentOS 7, and OpenNMS with the default configuration. I only added some nodes to the system (nothing else).

On the screenshot, you can see that the availability shown is negative for a node, even knowing that there are not outages for the node in question.

The negative availability appears as soon as the nodes are being properly categorized and dashboard is rendered.