Fix Popup.trackPointer Cannot read property 'classList' of undefined errors

[andrewharvey]

mapbox-gl-js version: 1.2.1

browser: n/a

Steps to Trigger Behavior

var popup = new mapboxgl.Popup({closeOnClick: false})
.setHTML('<h1>Hello World!</h1>')
.addTo(map);
  
popup.trackPointer();

ie call trackPointer without first setting a location or without first setting the popup contents.

Link to Demonstration

https://jsbin.com/jilulagivo/edit?html,output

Expected Behavior

Expected to handle this more gracefully without an error.

Actual Behavior

Cannot read property 'classList' of undefined

If anyone wants to pick this up by all means please do, otherwise I'll try to in a week or so.