Asset page shows error due to loading obsolete plot

[Flix6x]

This issue is related to #476. Since the asset page no longer shows a bokeh plot of the latest state (instead, an altair plot of sensor data is shown), the latest state plot should no longer be loaded by the server. For some assets, the asset page may show the following error message: ValueError : "Lightness must be between 0 and 1.".

[Flix6x]

I have only encountered the error when the asset's power sensor has no data yet.