check in a loop needed

[cekvenich]

This is my 'main' lib to use. When used standalone, works. Some polyfills, like web components, break it. So in my local copy I changed line 82 and 83 form:

for (i in listeners) { listeners[i].callback(data)

to:
for (i in listeners) { if(listeners[i].callback) listeners[i].callback(data)

Let me know if you would like the PR.