Fix navigation block off-canvas appender for empty menus (#48907)

- Pass the parent client id in as a prop instead of trying to determine the id from inner blocks

packages/block-editor/src/components/off-canvas-editor/appender.js

@@ -105,7 +105,7 @@ export const Appender = forwardRef(
 					ref={ ref }
 					rootClientId={ clientId }
 					position="bottom right"
-					isAppender={ true }
+					isAppender
 					selectBlockOnInsert={ false }
 					shouldDirectInsert={ false }
 					__experimentalIsQuick

packages/block-editor/src/components/off-canvas-editor/index.js

@@ -56,6 +56,7 @@ export const BLOCK_LIST_ITEM_HEIGHT = 36;
  *
  * @param {Object}  props                 Components props.
  * @param {string}  props.id              An HTML element id for the root element of ListView.
+ * @param {string}  props.parentClientId  The client id of the parent block.
  * @param {Array}   props.blocks          Custom subset of block client IDs to be used instead of the default hierarchy.
  * @param {boolean} props.showBlockMovers Flag to enable block movers
  * @param {boolean} props.isExpanded      Flag to determine whether nested levels are expanded by default.
@@ -68,6 +69,7 @@ export const BLOCK_LIST_ITEM_HEIGHT = 36;
 function OffCanvasEditor(
 	{
 		id,
+		parentClientId,
 		blocks,
 		showBlockMovers = false,
 		isExpanded = false,
@@ -229,6 +231,7 @@ function OffCanvasEditor(
 				>
 					<ListViewContext.Provider value={ contextValue }>
 						<ListViewBranch
+							parentId={ parentClientId }
 							blocks={ clientIdsTree }
 							selectBlock={ selectEditorBlock }
 							showBlockMovers={ showBlockMovers }

packages/block-library/src/navigation/edit/menu-inspector-controls.js

@@ -69,6 +69,7 @@ const MainContent = ( {
 	return (
 		<OffCanvasEditor
 			blocks={ clientIdsTree }
+			parentClientId={ clientId }
 			isExpanded={ true }
 			LeafMoreMenu={ LeafMoreMenu }
 			description={ description }