Removing the first hidden toolbar button throws an exception

[mlewand]

📝 Provide detailed reproduction steps (if any)

1. Go to https://ckeditor.com/docs/ckeditor5/20.0.0/examples/builds/document-editor.html
2. Resize the window in a way that at least one button gets hidden.

1. Use the following code to remove the first hidden button:

const toolbarView = editor.ui.view.toolbar;
const visibleButtonsCount = toolbarView._behavior.ungroupedItems.length;
const firstHiddenItem = toolbarView.items.get( visibleButtonsCount );
toolbarView.items.remove( firstHiddenItem );

✔️ Expected result

The first hidden toolbar button gets removed, and (if possible) one of hidden buttons gets "promoted" to the toolbar button.

❌ Actual result

An error is thrown:

Uncaught CKEditorError: collection-remove-404: Item not found. Read more: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/support/error-codes.html#error-collection-remove-404

    at ViewCollection._remove (http://localhost:8125/ckeditor5/tests/manual/all-features.js:143318:10)
    at ViewCollection.remove (http://localhost:8125/ckeditor5/tests/manual/all-features.js:142909:32)
    at ViewCollection.<anonymous> (http://localhost:8125/ckeditor5/tests/manual/all-features.js:138818:25)
    at ViewCollection.fire (http://localhost:8125/ckeditor5/tests/manual/all-features.js:146827:30)
    at ViewCollection._remove (http://localhost:8125/ckeditor5/tests/manual/all-features.js:143328:8)
    at ViewCollection.remove (http://localhost:8125/ckeditor5/tests/manual/all-features.js:142909:32)
    at <anonymous>:1:19

📃 Other details

That's a typical OBOE originating in this line:

ckeditor5/packages/ckeditor5-ui/src/toolbar/toolbarview.js

Line 567 in 8f3c81b

if ( index > this.ungroupedItems.length ) {

It's enough to change it to if ( index >= this.ungroupedItems.length ) {

There's a similar situation with adding buttons

ckeditor5/packages/ckeditor5-ui/src/toolbar/toolbarview.js

Line 553 in 8f3c81b

if ( index > this.ungroupedItems.length ) {

but it doesn't throw an error, instead _updateGrouping() call that follows it cleans up the structure, and no problem is visible.

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