When updating the dataset associated with an existing Handlebar chart, all the previous HTML and CSS gets lost

[geido]

Bug description

When you have an existing Handlebar chart with personalized HTML and CSS code, if you update the dataset that powers the chart, all the previous HTML and CSS get removed.

How to reproduce the bug

1. Create a virtual dataset with something simple like select * from YourTable
2. Create a handlebar chart with that dataset and add some HTML and CSS code in the customize section
3. From the Chart section, click on the 3 dots on the left and “edit dataset”
4. Change the query by adding a where clause or any other change you want.
5. Save the dataset and click on “Update Chart”

Expected results

Your dataset gets updated and you have the same previous HTML and CSS applied to your handlebars chart

Actual results

Your dataset gets updated but all the previous HTML and CSS code you had gets lost. It also seems that the textarea "blinks" going back and forth between states.

Screenshots/recordings

Superset version

master / latest-dev

Python version

3.9

Node version

18 or greater

Browser

Chrome

Additional context

No response

Checklist

• I have searched Superset docs and Slack and didn't find a solution to my problem.
• I have searched the GitHub issue tracker and didn't find a similar bug report.
• I have checked Superset's logs for errors and if I found a relevant Python stacktrace, I included it here as text in the "additional context" section.

[geido]

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[DamianPendrak]

Hi, could I work on this?