Helpful assertion for isAlwaysShown error (#58258)

packages/flutter/lib/src/cupertino/scrollbar.dart

@@ -62,7 +62,8 @@ class CupertinoScrollbar extends StatefulWidget {
     this.controller,
     this.isAlwaysShown = false,
     @required this.child,
-  }) : super(key: key);
+  }) : assert(!isAlwaysShown || controller != null, 'When isAlwaysShown is true, must pass a controller that is attached to a scroll view'),
+       super(key: key);
 
   /// The subtree to place inside the [CupertinoScrollbar].
   ///
@@ -276,7 +277,6 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
   void _triggerScrollbar() {
     WidgetsBinding.instance.addPostFrameCallback((Duration duration) {
       if (widget.isAlwaysShown) {
-        assert(widget.controller != null);
         _fadeoutTimer?.cancel();
         widget.controller.position.didUpdateScrollPositionBy(0);
       }

packages/flutter/lib/src/material/scrollbar.dart

@@ -38,7 +38,8 @@ class Scrollbar extends StatefulWidget {
     @required this.child,
     this.controller,
     this.isAlwaysShown = false,
-  }) : super(key: key);
+  }) : assert(!isAlwaysShown || controller != null, 'When isAlwaysShown is true, must pass a controller that is attached to a scroll view'),
+       super(key: key);
 
   /// The widget below this widget in the tree.
   ///
@@ -131,7 +132,6 @@ class _ScrollbarState extends State<Scrollbar> with TickerProviderStateMixin {
   void _triggerScrollbar() {
     WidgetsBinding.instance.addPostFrameCallback((Duration duration) {
       if (widget.isAlwaysShown) {
-        assert(widget.controller != null);
         _fadeoutTimer?.cancel();
         widget.controller.position.didUpdateScrollPositionBy(0);
       }

packages/flutter/test/cupertino/scrollbar_test.dart

@@ -164,6 +164,58 @@ void main() {
     await tester.pump(_kScrollbarFadeDuration);
   });
 
+  testWidgets('When isAlwaysShown is true, must pass a controller',
+      (WidgetTester tester) async {
+    Widget viewWithScroll() {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: CupertinoScrollbar(
+            isAlwaysShown: true,
+            child: const SingleChildScrollView(
+              child: SizedBox(
+                width: 4000.0,
+                height: 4000.0,
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    expect(() async {
+      await tester.pumpWidget(viewWithScroll());
+    }, throwsAssertionError);
+  });
+
+  testWidgets('When isAlwaysShown is true, must pass a controller that is attached to a scroll view',
+      (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    Widget viewWithScroll() {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: CupertinoScrollbar(
+            controller: controller,
+            isAlwaysShown: true,
+            child: const SingleChildScrollView(
+              child: SizedBox(
+                width: 4000.0,
+                height: 4000.0,
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(viewWithScroll());
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
+  });
+
   testWidgets('On first render with isAlwaysShown: true, the thumb shows',
       (WidgetTester tester) async {
     final ScrollController controller = ScrollController();

packages/flutter/test/material/scrollbar_test.dart

@@ -201,6 +201,56 @@ void main() {
     expect(scrollbar.controller, isNotNull);
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
 
+  testWidgets('When isAlwaysShown is true, must pass a controller',
+      (WidgetTester tester) async {
+    Widget viewWithScroll() {
+      return _buildBoilerplate(
+        child: Theme(
+          data: ThemeData(),
+          child: Scrollbar(
+            isAlwaysShown: true,
+            child: const SingleChildScrollView(
+              child: SizedBox(
+                width: 4000.0,
+                height: 4000.0,
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    expect(() async {
+      await tester.pumpWidget(viewWithScroll());
+    }, throwsAssertionError);
+  });
+
+  testWidgets('When isAlwaysShown is true, must pass a controller that is attached to a scroll view',
+      (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    Widget viewWithScroll() {
+      return _buildBoilerplate(
+        child: Theme(
+          data: ThemeData(),
+          child: Scrollbar(
+            isAlwaysShown: true,
+            controller: controller,
+            child: const SingleChildScrollView(
+              child: SizedBox(
+                width: 4000.0,
+                height: 4000.0,
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(viewWithScroll());
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
+  });
+
   testWidgets('On first render with isAlwaysShown: true, the thumb shows',
       (WidgetTester tester) async {
     final ScrollController controller = ScrollController();