examples/web/async.py: selecting item clears dropdown list

[otaku42]

Said example provides several dropdown lists, for menu controls and for allowing to select a resolution and format. When selecting one of the values of a dropdown list, after the value gets applied to the corresponding control that dropdown list then gets emptied completely. One has to either reload the page or to switch to a different camera and back (given that more than one camera is available).

@tiagocoutinho: I've already tried to debug this issue, but have failed. I'm willing to give it another try, but would need a pointer to how and where this bug might be caused.

[otaku42]

By the way, I've tested with Firefox 97.0 and Chromium 113.0.5672.63, the behaviour occurs in both browsers.