Fix highchart error on update before mounted.

[dclause]

Open the page: https://nicegui.io/reference#chart with your console opened.

You will see an error :
Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'series')

This is because the chart.py component is updated without the vue component actually mounted yet, hence the chart does not exists and can be updated.
Plus, if the component unmount/remount on the life of a single page, the chart is therefore leaking. Added a destroy method to handle that.

[falkoschindler]

Thanks for the PR, @dclause!
You're certainly right. But I can't reproduce the error. Which console do you mean, Python or JS?

[dclause]

Chrome console:

• open your chrome devtool console with F12
• clic on that direct link : https://nicegui.io/reference#chart
• see the erorr:
  

[falkoschindler]

Sorry, I think I mixed up ui.chart with ui.plotly. Now that I looked at the correct example, I got the error in the JS console. And I could reproduce the issue locally with throttling the network to "Fast 3G".

Your code looks good and resolves the error. Thanks again! Let's merge it into main. 🙂