Using Sentry to report user exceptions, I get a lot of the following error:

null is not an object (evaluating 'this._transport.transportName')

I believe it's coming from:

I have a separate issue where I'm unable to use the minified version due to #393. Since I'm using the non-minified version, the debug() calls are still there in the code and cause this issue.

It looks like some kind of a race condition where the connection is closed before _open() is called. It's only the next line that checks for that case. The debug() call assumes the connection is open and accesses _transport like it's open.

I'm not sure what the best solution for this would be, but in my case I just commented out that line and it seems to resolve the issue.