fix(tab): avoid TabController crash when all conditional tabs are hidden

modules/ensemble/lib/layout/tab/scrollable_tab_bar.dart

@@ -114,14 +114,19 @@ class ScrollableTabBarState extends BaseTabBarState {
   }
 
   void _initializeTabController() {
+    final tabLength =
+        effectiveTabControllerLength(widget.controller.items.length);
     tabController = TabController(
-      length: widget.controller.items.length,
+      length: tabLength,
       vsync: this,
     );
   }
 
   void _reinitializeTabController() {
     tabController.dispose();
+    if (widget._controller.selectedIndex >= widget._controller.items.length) {
+      widget._controller.selectedIndex = 0;
+    }
     _initializeTabController();
   }
 

modules/ensemble/lib/layout/tab/tab_bar_controller.dart

@@ -6,8 +6,17 @@ import 'package:ensemble/layout/tab_bar.dart';
 import 'package:ensemble/util/utils.dart';
 import 'package:ensemble/widget/helpers/controllers.dart';
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
 import 'package:yaml/yaml.dart';
 
+/// [TabController] requires `length >= 1`. When conditional `visible` bindings
+/// hide every tab, the UI shows nothing but the controller still needs a valid length.
+@visibleForTesting
+int effectiveTabControllerLength(int visibleTabCount) {
+  assert(visibleTabCount >= 0);
+  return visibleTabCount == 0 ? 1 : visibleTabCount;
+}
+
 // the Controller for the TabBar
 class TabBarController extends BoxController {
   String? tabPosition;

modules/ensemble/lib/layout/tab_bar.dart

@@ -113,12 +113,17 @@ class TabBarState extends BaseTabBarState {
     if (widget.controller.useIndexedTab) {
       _cache = List<Widget?>.filled(widget.controller.items.length, null);
     }
+    final tabLength =
+        effectiveTabControllerLength(widget.controller.items.length);
     tabController = TabController(
-      initialIndex: safeIndex,
-      length: widget.controller.items.length,
+      initialIndex:
+          widget.controller.items.isEmpty ? 0 : safeIndex.clamp(0, tabLength - 1),
+      length: tabLength,
       vsync: this,
     );
-    tabController.addListener(notifyListener);
+    if (widget.controller.items.isNotEmpty) {
+      tabController.addListener(notifyListener);
+    }
   }
 
   int _getValidInitialIndex() {

modules/ensemble/test/effective_tab_controller_length_test.dart

@@ -0,0 +1,15 @@
+import 'package:ensemble/layout/tab/tab_bar_controller.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('effectiveTabControllerLength', () {
+    test('uses placeholder length 1 when no tabs are visible', () {
+      expect(effectiveTabControllerLength(0), 1);
+    });
+
+    test('matches visible tab count when at least one tab is visible', () {
+      expect(effectiveTabControllerLength(1), 1);
+      expect(effectiveTabControllerLength(4), 4);
+    });
+  });
+}