fix no room selected after leave / init

lib/controllers/selected_room.dart

@@ -0,0 +1,31 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/key.dart";
+import "package:nexus/controllers/selected_space.dart";
+import "package:nexus/helpers/extensions/get_all_child_rooms.dart";
+
+class SelectedRoomController extends Notifier<String?> {
+  @override
+  String? build() {
+    final roomId = ref.watch(
+      KeyController.provider(KeyController.roomKey)
+          .select((async) => async.value),
+    );
+
+    final roomIds = ref.watch(
+      SelectedSpaceController.provider.select(
+        (space) => space?.allChildRooms
+            .map((room) => room.metadata?.id)
+            .nonNulls
+            .toIList(),
+      ),
+    );
+    if (roomIds == null) return roomId;
+
+    return roomIds.contains(roomId) ? roomId : roomIds.firstOrNull;
+  }
+
+  static final provider = NotifierProvider<SelectedRoomController, String?>(
+    SelectedRoomController.new,
+  );
+}