native dragover handler is assigned invalid handler and can throw in some situations

[cmawhorter]

with these lines, this is an instance of Sortable.

https://github.com/SortableJS/Sortable/blob/daaefedad3c59204edfdbb27706e1447e6eb41d8/src/Sortable.js#L432-L433

i believe it's supposed to be:

    on(el, 'dragover', this._onDragOver.bind(this));
    on(el, 'dragenter', this._onDragOver.bind(this));

this can sometimes lead to this error getting thrown. ...though i don't understand why it doesn't always throw that error and still work most of the time?