Widgets editor - can't add or save

[Simonsbs]

I'm trying to add a block to a widget area - but no matter what I do I get an error "There was an error. [object Object]"

To reproduce
Steps to reproduce the behavior:

1. Go to Design -> Widgets
2. Add a block to any area
3. Click Save
4. See an error in the bottom corner

Expected behavior
Save the changes made

Screenshots

Editor version (please complete the following information):

• WordPress version: 5.6
• using the block editor that comes by default

Desktop (please complete the following information):

• OS: Windows 10
• Browser: chrome

[bph]

Is there an error message in the browser console?
3-dot menu > More Tools > Developer Tools

The first tap is the "Console". It'll help to clear all previous error messages and refresh the page.

[Soean]

I can reproduce the error, if I add a block to an empty widget area:

Request payload:

{
   "id":"sidebar-1",
   "widgets":[
      null
   ]
}

Response:

{
   "code":"rest_invalid_param",
   "message":"Invalid parameter(s): widgets",
   "data":{
      "status":400,
      "params":{
         "widgets":"widgets[0] is not of type object,string."
      }
   }
}

[annezazu]

Ran into this today with Twenty Twenty theme and 9.6.2: https://cloudup.com/cdA49pgrb0N Marking this as a regression and adding to the project board for widgets to get more attention!

[annezazu]

@tellthemachines kindly pointed out to me that this is a duplicate of #27173 Going to close this out for now and defer to the original issue which has a bit more information.