diff --git a/lib/main.dart b/lib/main.dart
index bf926dc..c041243 100644
--- a/lib/main.dart
+++ b/lib/main.dart
@@ -68,8 +68,12 @@ void showGlobalSnackBar(String message, {bool isError = false}) {
 
 /// Show a dialog safely through the global navigator key.
 Future<T?> showGlobalDialog<T>({required WidgetBuilder builder}) {
+  final context = navigatorKey.currentContext;
+  if (context == null) {
+    return Future<T?>.value(null);
+  }
   return showDialog<T>(
-    context: navigatorKey.currentContext!,
+    context: context,
     builder: builder,
   );
 }
@@ -201,8 +205,14 @@ class _AuthGateState extends State<AuthGate> {
   }
 
   Future<void> _showResetPasswordDialog() async {
+    final context = navigatorKey.currentContext;
+    if (context == null) {
+      _isInPasswordRecoveryFlow = false;
+      return;
+    }
+
     await showDialog<void>(
-      context: navigatorKey.currentContext!,
+      context: context,
       barrierDismissible: false,
       builder: (_) => const _ResetPasswordDialog(),
     );
diff --git a/lib/screens/garage_screen.dart b/lib/screens/garage_screen.dart
index 659d808..cb59788 100644
--- a/lib/screens/garage_screen.dart
+++ b/lib/screens/garage_screen.dart
@@ -483,6 +483,11 @@ class GarageScreenState extends State<GarageScreen> {
             .update({'collection_id': targetId})
             .inFilter('id', _selectedIds.toList());
       } else {
+        final userId = supabase.auth.currentUser?.id;
+        if (userId == null) {
+          throw Exception('You must be signed in to copy cars.');
+        }
+
         final sourceCars = _cars
             .where((car) => _selectedIds.contains(car['id'] as int))
             .toList(growable: false);
@@ -492,7 +497,7 @@ class GarageScreenState extends State<GarageScreen> {
           final imagePath = car['user_image_url'] as String?;
           return <String, dynamic>{
             'hw_id': car['hw_id'] as String,
-            'user_id': supabase.auth.currentUser!.id,
+            'user_id': userId,
             'collection_id': targetId,
             if (notes != null && notes.trim().isNotEmpty) 'notes': notes,
             if (imagePath != null && imagePath.isNotEmpty)
@@ -1028,11 +1033,16 @@ class GarageScreenState extends State<GarageScreen> {
             .update({'collection_id': targetId})
             .eq('id', car['id']);
       } else {
+        final userId = supabase.auth.currentUser?.id;
+        if (userId == null) {
+          throw Exception('You must be signed in to copy cars.');
+        }
+
         final notes = car['notes'] as String?;
         final imagePath = car['user_image_url'] as String?;
         await supabase.from('hotwheels').insert({
           'hw_id': car['hw_id'] as String,
-          'user_id': supabase.auth.currentUser!.id,
+          'user_id': userId,
           'collection_id': targetId,
           if (notes != null && notes.trim().isNotEmpty) 'notes': notes,
           if (imagePath != null && imagePath.isNotEmpty)
diff --git a/lib/screens/scan_tab.dart b/lib/screens/scan_tab.dart
index 778d243..90c2f30 100644
--- a/lib/screens/scan_tab.dart
+++ b/lib/screens/scan_tab.dart
@@ -419,9 +419,14 @@ class ScanTabState extends State<ScanTab> {
     String hwId, {
     String? notes,
   }) async {
+    final userId = supabase.auth.currentUser?.id;
+    if (userId == null) {
+      throw Exception('You must be signed in to add cars.');
+    }
+
     await supabase.from('hotwheels').insert({
       'hw_id': hwId,
-      'user_id': supabase.auth.currentUser!.id,
+      'user_id': userId,
       'collection_id': collectionId,
       if (notes != null && notes.trim().isNotEmpty) 'notes': notes.trim(),
     });
@@ -433,6 +438,11 @@ class ScanTabState extends State<ScanTab> {
     String? series,
     int? year,
   }) async {
+    final userId = supabase.auth.currentUser?.id;
+    if (userId == null) {
+      throw Exception('You must be signed in to create catalog entries.');
+    }
+
     final cleanedSeries = series?.trim();
     final payload = <String, dynamic>{
       'hw_id': hwId,
@@ -445,12 +455,15 @@ class ScanTabState extends State<ScanTab> {
 
     await supabase.from('car_votes').insert({
       'hw_id': hwId,
-      'user_id': supabase.auth.currentUser!.id,
+      'user_id': userId,
     });
   }
 
   Future<void> _ensureValidationVote(String hwId) async {
-    final userId = supabase.auth.currentUser!.id;
+    final userId = supabase.auth.currentUser?.id;
+    if (userId == null) {
+      throw Exception('You must be signed in to validate entries.');
+    }
     final existingVote = await supabase
         .from('car_votes')
         .select('id')
diff --git a/lib/services/collection_service.dart b/lib/services/collection_service.dart
index a38105c..088b914 100644
--- a/lib/services/collection_service.dart
+++ b/lib/services/collection_service.dart
@@ -53,6 +53,14 @@ class CollectionMember {
 class CollectionService {
   CollectionService._();
 
+  static String _requireUserId() {
+    final userId = supabase.auth.currentUser?.id;
+    if (userId == null) {
+      throw Exception('You must be signed in to perform this action.');
+    }
+    return userId;
+  }
+
   /// Ensures the current user has at least one collection membership.
   /// Creates a default collection on first login.
   static Future<String?> ensureDefaultCollection() async {
@@ -80,7 +88,7 @@ class CollectionService {
   /// Fetch all collections the current user is a member of,
   /// including item count and member count.
   static Future<List<Collection>> getMyCollections() async {
-    final userId = supabase.auth.currentUser!.id;
+    final userId = _requireUserId();
 
     // Get memberships with collection data.
     final memberships = await supabase
@@ -249,7 +257,7 @@ class CollectionService {
     required String name,
     String? description,
   }) async {
-    final userId = supabase.auth.currentUser!.id;
+    final userId = _requireUserId();
 
     final row = await supabase
         .from('collections')
@@ -287,7 +295,7 @@ class CollectionService {
     required String name,
     String? description,
   }) async {
-    final userId = supabase.auth.currentUser!.id;
+    final userId = _requireUserId();
 
     final collection = await supabase
         .from('collections')
@@ -311,7 +319,7 @@ class CollectionService {
 
   /// Delete a collection. Owner only. Cascade deletes members & items.
   static Future<void> delete(String collectionId) async {
-    final userId = supabase.auth.currentUser!.id;
+    final userId = _requireUserId();
 
     final collection = await supabase
         .from('collections')
@@ -362,7 +370,7 @@ class CollectionService {
       throw Exception('Unsupported role "$role".');
     }
 
-    final currentUserId = supabase.auth.currentUser!.id;
+    final currentUserId = _requireUserId();
 
     final collection = await supabase
         .from('collections')
@@ -428,7 +436,7 @@ class CollectionService {
     required String collectionId,
     required String memberUserId,
   }) async {
-    final currentUserId = supabase.auth.currentUser!.id;
+    final currentUserId = _requireUserId();
 
     final collection = await supabase
         .from('collections')
@@ -501,7 +509,7 @@ class CollectionService {
 
   /// Leave a collection (for non-owners).
   static Future<void> leave(String collectionId) async {
-    final userId = supabase.auth.currentUser!.id;
+    final userId = _requireUserId();
 
     final membership = await supabase
         .from('collection_members')
diff --git a/lib/services/storage_service.dart b/lib/services/storage_service.dart
index e07b9d9..c437afc 100644
--- a/lib/services/storage_service.dart
+++ b/lib/services/storage_service.dart
@@ -31,7 +31,11 @@ class StorageService {
       await deleteCarImage(oldPath);
     }
 
-    final userId = supabase.auth.currentUser!.id;
+    final user = supabase.auth.currentUser;
+    if (user == null) {
+      throw Exception('You must be signed in to upload images.');
+    }
+    final userId = user.id;
     final path = '$userId/$entryId.jpg';
     final compressed = await _compressImage(file);