Error log when I call webgazer.begin()

I am using webgazer to do some eye tracking experiment, and find every time there is an error log in terminal.

Start with the reported webgazer.begin() call and reproduce the terminal error using the attached output. Trace that log to the relevant WebGazer entry point and determine whether it is expected or indicates a failure. Done means the cause is identified and the unwanted error log no longer appears under the reported setup.

Written by the indexing model from the issue text.