multiple times in listener.length can not exist when object does not exist either

[designerfuzzi]

https://github.com/pjb493/MIDIWebView/blob/793edd9d37a89e58ecbe285fec00818d009d436a/MIDIWebViewPolyfill/src/MIDIEventDispatcher.js#L8

this particular line of code is multiple times in your lib and will certainly fail.
The encapsulated for loop and so on can not expose listener.length when there is no listener object ..
guess shall be easy fixed to

const listener = this._listener[index]
if (listener) {
    for (var i=0; i< listener.length; ++i) {
        //do something here
    }
}