Widgets registering multpile instances don't display in available widgets list.

[mdawaffe]

When widgets use the new-fangled "multi-widget" code, corrupt or un-upgraded data can sometimes prevent any widgets from showing. This is due to a bad "are there any widgets of this type" check in WordPress core.

Attached fixes that problem.

When widgets use the old-fangled "how many of these widgets do you want to use: 1,2,..." UI and then register that many widgets, only the first instance is accessible.

Attached does not fix that problem. Debugging now.

[mdawaffe]

fix new-fangled multi widgets

[ryan]

[7077]

[mdawaffe]

old-fangled

[mdawaffe]

6023-old.diff fixes the problem with the test widget I have. If you have a widget that doesn't work under this patch, please attach it to this ticket so I can debug some more.

[ryan]

(In [7080]) Fix widgets using old-style multi instance support. Props mdawaffe. fixes #6023

[westonruter]

In 61466:

Widgets: Ensure a widget is registered prior to checking its ID in is_active_widget().

This avoids a possible PHP warning due to an undefined array key.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/10710

Follow-up to [11090], [7080].

Props sageth, josephscott, Ipstenu, mindctrl, westonruter.
See #8441, #6023.
Fixes #57518.