[material_ui] Let TabBarThemeData.indicator satisfy the indicatorWeight assert

packages/material_ui/lib/src/tabs.dart

@@ -1066,8 +1066,7 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
     this.tabAlignment,
     this.textScaler,
     this.indicatorAnimation,
-  }) : _isPrimary = true,
-       assert(indicator != null || (indicatorWeight > 0.0));
+  }) : _isPrimary = true;
 
   /// Creates a Material Design secondary tab bar.
   ///
@@ -1128,8 +1127,7 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
     this.tabAlignment,
     this.textScaler,
     this.indicatorAnimation,
-  }) : _isPrimary = false,
-       assert(indicator != null || (indicatorWeight > 0.0));
+  }) : _isPrimary = false;
 
   /// Typically a list of two or more [Tab] widgets.
   ///
@@ -1173,7 +1171,8 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
 
   /// The thickness of the line that appears below the selected tab.
   ///
-  /// The value of this parameter must be greater than zero.
+  /// The value of this parameter must be greater than zero, unless an indicator
+  /// is provided by [indicator] or [TabBarThemeData.indicator].
   ///
   /// If [ThemeData.useMaterial3] is true and [TabBar] is used to create a
   /// primary tab bar, the default value is 3.0. If the provided value is less
@@ -1645,6 +1644,13 @@ class _TabBarState extends State<TabBar> {
       return tabBarTheme.indicator!;
     }
 
+    assert(
+      widget.indicatorWeight > 0.0,
+      'The indicatorWeight must be greater than zero when the TabBar draws its '
+      'default underline indicator, i.e. when no indicator is provided by the '
+      'TabBar or by TabBarThemeData.indicator.',
+    );
+
     Color color = widget.indicatorColor ?? tabBarTheme.indicatorColor ?? _defaults.indicatorColor!;
     // ThemeData tries to avoid this by having indicatorColor avoid being the
     // primaryColor. However, it's possible that the tab bar is on a

packages/material_ui/pending_changelogs/change_2026_08_28_tab_bar_indicator_weight.yaml

@@ -0,0 +1,3 @@
+changelog: |
+  - Fixes `TabBar` asserting on construction when `indicatorWeight` is zero and the indicator is provided by `TabBarThemeData.indicator`.
+version: patch

packages/material_ui/test/tabs_test.dart

@@ -3160,6 +3160,75 @@ void main() {
     );
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/188837.
+  testWidgets('TabBar.indicatorWeight can be zero when the indicator comes from the theme', (
+    WidgetTester tester,
+  ) async {
+    const indicatorColor = Color(0xFF00FF00);
+    const tabBarTheme = TabBarThemeData(
+      indicator: BoxDecoration(color: indicatorColor),
+      indicatorSize: TabBarIndicatorSize.tab,
+    );
+
+    final tabs = List<Widget>.generate(2, (int index) => Tab(text: 'Tab $index'));
+
+    Widget buildTabBar({bool secondaryTabBar = false}) {
+      final TabController controller = createTabController(
+        vsync: const TestVSync(),
+        length: tabs.length,
+      );
+      return boilerplate(
+        useMaterial3: false,
+        tabBarTheme: tabBarTheme,
+        child: Container(
+          alignment: Alignment.topLeft,
+          child: secondaryTabBar
+              ? TabBar.secondary(indicatorWeight: 0.0, controller: controller, tabs: tabs)
+              : TabBar(indicatorWeight: 0.0, controller: controller, tabs: tabs),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildTabBar());
+    expect(tester.takeException(), isNull);
+
+    RenderBox tabBarBox = tester.firstRenderObject<RenderBox>(find.byType(TabBar));
+    // 46 = _kTabHeight(46) + indicatorWeight(0.0)
+    expect(tabBarBox.size.height, 46.0);
+    expect(
+      tabBarBox,
+      paints..rect(rect: const Rect.fromLTRB(0.0, 0.0, 400.0, 46.0), color: indicatorColor),
+    );
+
+    await tester.pumpWidget(buildTabBar(secondaryTabBar: true));
+    expect(tester.takeException(), isNull);
+
+    tabBarBox = tester.firstRenderObject<RenderBox>(find.byType(TabBar));
+    expect(tabBarBox.size.height, 46.0);
+    expect(
+      tabBarBox,
+      paints..rect(rect: const Rect.fromLTRB(0.0, 0.0, 400.0, 46.0), color: indicatorColor),
+    );
+  });
+
+  testWidgets('TabBar asserts when indicatorWeight is zero and no indicator is provided', (
+    WidgetTester tester,
+  ) async {
+    final tabs = List<Widget>.generate(2, (int index) => Tab(text: 'Tab $index'));
+    final TabController controller = createTabController(
+      vsync: const TestVSync(),
+      length: tabs.length,
+    );
+
+    await tester.pumpWidget(
+      boilerplate(
+        child: TabBar(indicatorWeight: 0.0, controller: controller, tabs: tabs),
+      ),
+    );
+
+    expect(tester.takeException(), isAssertionError);
+  });
+
   testWidgets('TabBar with indicatorWeight, indicatorPadding (LTR)', (WidgetTester tester) async {
     const indicatorColor = Color(0xFF00FF00);
     const indicatorWeight = 8.0;