Avoid getTheme call when slug is null (#95570)

apps/wpcom-block-editor/src/wpcom/features/live-preview/hooks/use-previewing-theme.ts

@@ -45,6 +45,13 @@ export const usePreviewingTheme = () => {
 	const previewingThemeSlug = usePreviewingThemeSlug();
 	const { previewingThemeName } = useSelect(
 		( select ) => {
+			// Avoid calling getTheme with null - fetches all site /themes (10s~)
+			if ( ! previewingThemeSlug ) {
+				return {
+					previewingThemeSlug,
+					previewingThemeName: undefined,
+				};
+			}
 			// eslint-disable-next-line @typescript-eslint/no-explicit-any
 			const previewingTheme = ( select( 'core' ) as any ).getTheme( previewingThemeSlug );
 

apps/wpcom-block-editor/src/wpcom/features/live-preview/index.tsx

@@ -1,16 +1,23 @@
 import { useSelect } from '@wordpress/data';
 import domReady from '@wordpress/dom-ready';
 import { registerPlugin } from '@wordpress/plugins';
+import { usePreviewingThemeSlug } from './hooks/use-previewing-theme';
 import LivePreviewNoticePlugin from './live-preview-notice-plugin';
 
 const LivePreviewPlugin = () => {
 	const siteEditorStore = useSelect( ( select ) => select( 'core/edit-site' ), [] );
+	const previewingThemeSlug = usePreviewingThemeSlug();
 
 	// Do nothing outside the Site Editor context.
 	if ( ! siteEditorStore ) {
 		return null;
 	}
 
+	// Don't render unless the user is previewing a theme.
+	if ( ! previewingThemeSlug ) {
+		return null;
+	}
+
 	return <LivePreviewNoticePlugin />;
 };