Fix usages of uSES with missing getServerSnapshot (#60943)

* Fix usages of uSES with missing getServerSnapshot

* Fill PR number in changelog

Author: jsnajdr

packages/components/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Bug Fix
+
+-   `SlotFill`: fixed missing `getServerSnapshot` parameter in slot map ([#60943](https://github.com/WordPress/gutenberg/pull/60943)).
+
 ## 27.4.0 (2024-04-19)
 
 ### Deprecation

packages/components/src/slot-fill/bubbles-virtually/observable-map.ts

@@ -75,5 +75,5 @@ export function useObservableValue< K, V >(
 		],
 		[ map, name ]
 	);
-	return useSyncExternalStore( subscribe, getValue );
+	return useSyncExternalStore( subscribe, getValue, getValue );
 }

packages/plugins/src/components/plugin-area/index.tsx

@@ -113,7 +113,11 @@ function PluginArea( {
 		};
 	}, [ scope ] );
 
-	const plugins = useSyncExternalStore( store.subscribe, store.getValue );
+	const plugins = useSyncExternalStore(
+		store.subscribe,
+		store.getValue,
+		store.getValue
+	);
 
 	return (
 		<div style={ { display: 'none' } }>