Customizer: Error in console after opening a widget area when a theme opts-out of the new block-based “Widgets” screen

[pagelab]

Describe the bug
Since Gutenberg 8.9.0 RC1 (#24087) it is possible to opt-out of the new block-based “Widgets” screen adding remove_theme_support( 'widgets-block-editor' ); to any theme.

This keeps the legacy “Widgets” screen and prevent the new block-based “Widgets” screen to appear.

But doing so does not bring back the legacy “Widgets” section on the customizer.

It's crucial to maintain this section for backward compatibility reasons, specially for themes that heavily use widgets for layout purposes or widgets that have many visual controls, like color pickers.

To reproduce
Steps to reproduce the behavior:

1. Using any theme, add remove_theme_support( 'widgets-block-editor' ); in the functions.php file to opt-out of the new block-based “Widgets” screen.
2. Install and activate the Gutenberg 8.9.0 RC1 plugin in “Plugins → Add new”.
3. Click “Appearance → Customize”.
4. See that the legacy “Widgets” section is missing.

Expected behavior
When a theme opts-out of the new block-based “Widgets” screen, the legacy “Widgets” section should appear on the customizer.

Screenshots

Editor version:

• WordPress version: 5.5.0
• Gutenberg 8.9.0 RC1
• TwentyTwenty 1.5

Desktop:

• macOS 10.14.6
• Chrome 85

[adamziel]

This is temporarily fixed by merging #25626, although the discussion about the fate of the customizer editor is still ongoing: #25625. I'm marking it as blocked for now.

[bph]

I tested this with master today. (Oct 15, 2020)
The Widget section appears in the Customizer and I could add and delete widgets without any problems.

[pagelab]

I did a test with Gutenberg version 9.2.2 and it's working now.

[draganescu]

I am reopening this as trying to test with the Gutenberg plugin activated, adding remove_theme_support( 'widgets-block-editor' ); will result in Uncaught Error: Call to undefined method stdClass::_register() in wp-includes/class-wp-widget-factory.php:102.

[noisysocks]

@draganescu: Where are you seeing that error? When I enable the Gutenberg plugin, enable the Widgets screen in Customizer experiment, add remove_theme_support( 'widgets-block-editor' ); to gutenberg.php, and browse to Appearance → Customize → Widgets I see the old interface and no error.

[draganescu]

That was a while ago @noisysocks
Tested again, now there is no more PHP error, however, with the block widgets disabled from the theme functions file, I get the widgets panel back but currently on trunk opening that section or adding an image widget results in Uncaught TypeError: t.onChangeSectionExpanded is not a function

[tellthemachines]

I can partially reproduce this by adding remove_theme_support( 'widgets-block-editor' ); to my theme's functions.php. I can access the Widgets section in the Customizer but attempting to open any of the widgets for editing causes Uncaught TypeError: control.onChangeSectionExpanded is not a function to appear in the browser console.

[kevin940726]

remove_theme_support( 'widgets-block-editor' ) doesn't seem to work at all now. I opened trac-53424 to track it.

[getdave]

Looks like this was fixed in WordPress/wordpress-develop#1382.

More details in Trac.

https://core.trac.wordpress.org/ticket/53424#ticket

Can we close this one out now @noisysocks?