[6.x] Fix card designer preview element type

CHANGELOG.md

@@ -50,6 +50,7 @@
 - Fixed a bug where pressing <kbd>Ctrl/Cmd</kbd> + <kbd>S</kbd> in a slideout could also save the page behind it. ([#19705](https://github.com/craftcms/cms/pull/19705))
 - Fixed a bug where Shift-selecting a shorter range didn’t deselect items outside the new range. ([#19727](https://github.com/craftcms/cms/pull/19727))
 - Fixed a bug where select inputs could lose their selected value if their options were loaded asynchronously. ([#19728](https://github.com/craftcms/cms/pull/19728))
+- Fixed a bug where card designer previews could fail to render because the preview request omitted the element type. ([#19776](https://github.com/craftcms/cms/pull/19776))
 - Fixed a bug where reactive controls in element editors didn’t refresh their field layouts, including nested form scopes. ([#19769](https://github.com/craftcms/cms/pull/19769))
 
 ### Assets

resources/js/modules/field-layout-designer/card-view-designer.ts

@@ -135,6 +135,10 @@ export class CardViewDesigner extends Base {
           data: {
             fieldLayoutConfig: {
               ...this.designer.config,
+              // Not part of `this.designer.config` (that lives at
+              // `designer.settings.elementType` instead), but `CardDesigner::previewHtml()`
+              // needs a `type` on the layout config to instantiate a sample element.
+              type: this.designer.settings!.elementType,
               generatedFields:
                 document
                   .querySelector('craft-generated-fields-table')