optimization demo is partially broken?

[x-EricH-x]

when running the optimization.py locally, the page shows up correctly onload, but will break if click on different optimization runs.
(https://github.com/happydasch/btplotting/blob/master/demos/optimization.py)
(I did changed the port from default to 3000 by browser = BacktraderPlottingOptBrowser(btp, result, port=3000),
shouldn't matter I think?)

initial load screenshot:

broken state screenshot:

there is an error thrown in the brower dev console:

[x-EricH-x]

probably related
#54