Halfway markers error after read only toggle

[oliverfields]

Hi,

Great plugin!

I get an error when toggling read only and clicking on the halfway markers.

Reproduce:

1. Go http://scripter-co.github.io/leaflet-plotter/
2. Click Toggle Readonly twice
3. Click a halfway marker (the semi transparent ones)

Expected new marker to be created, but nothing happens.

My console reads:
TypeError: i is undefined
_removePoint http://scripter-co.github.io/leaflet-plotter/src/leaflet.plotter.js:85

Using Firefox.