Undefined error in plupload dispatchEvent on Mac S

问题:

I am experiencing weird errors on Mac Safari with plupload.js. The method dispatchEvent has this line:

evt.type = eventType;

It sometimes throws this error:

TypeError: undefined is not an object (evaluating 'evt.type = eventType')" userAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/602.3.12 (KHTML, like Gecko) Version/10.0.2 Safari/602.3.12

The evt object is declared at the top of the method:

var evt = {};

It is not re-declared anywhere below. I am not able to reproduce this error locally, I have it in my logs from other users. How can this happen? Might it be a problem with JS implementation on Mac Safari so that some other thread is erasing the evt object?

EDIT: The problematic line of code is here https://github.com/moxiecode/plupload/blob/3.x/js/plupload.js#L2200