Fix: Closing bottom sheet and removing FAB cause assertion failure (flutter#128566)

Fixes flutter#128562

Author: utisam

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

@@ -1358,11 +1358,9 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 
   void _handlePreviousAnimationStatusChanged(AnimationStatus status) {
     setState(() {
-      if (status == AnimationStatus.dismissed) {
+      if (widget.child != null && status == AnimationStatus.dismissed) {
         assert(widget.currentController.status == AnimationStatus.dismissed);
-        if (widget.child != null) {
-          widget.currentController.forward();
-        }
+        widget.currentController.forward();
       }
     });
   }

packages/flutter/test/material/scaffold_test.dart

@@ -2801,6 +2801,65 @@ void main() {
     // The scrim should be gone.
     expect(findModalBarrier(), findsNothing);
   });
+
+  testWidgets("Closing bottom sheet & removing FAB at the same time doesn't throw assertion", (WidgetTester tester) async {
+      final Key bottomSheetKey = UniqueKey();
+      PersistentBottomSheetController<void>? controller;
+      bool show = true;
+
+      await tester.pumpWidget(StatefulBuilder(
+        builder: (_, StateSetter setState) => MaterialApp(
+          home: Scaffold(
+            body: Center(
+            child: Builder(
+              builder: (BuildContext context) => ElevatedButton(
+                onPressed: () {
+                  if (controller == null) {
+                    controller = showBottomSheet(
+                      context: context,
+                      builder: (_) => Container(
+                        key: bottomSheetKey,
+                        height: 200,
+                      ),
+                    );
+                  } else {
+                    controller!.close();
+                    controller = null;
+                  }
+                },
+                child: const Text('BottomSheet'),
+              )),
+            ),
+            floatingActionButton: show
+              ? FloatingActionButton(onPressed: () => setState(() => show = false))
+              : null,
+          ),
+        ),
+      ));
+
+      // Show bottom sheet.
+      await tester.tap(find.byType(ElevatedButton));
+      await tester.pumpAndSettle(const Duration(seconds: 1));
+
+      // Bottom sheet and FAB are visible.
+      expect(find.byType(FloatingActionButton), findsOneWidget);
+      expect(find.byKey(bottomSheetKey), findsOneWidget);
+
+      // Close bottom sheet while removing FAB.
+      await tester.tap(find.byType(FloatingActionButton));
+      await tester.pump(); // start animation
+      await tester.tap(find.byType(ElevatedButton));
+      // Let the animation finish.
+      await tester.pumpAndSettle(const Duration(seconds: 1));
+
+      // Bottom sheet and FAB are gone.
+      expect(find.byType(FloatingActionButton), findsNothing);
+      expect(find.byKey(bottomSheetKey), findsNothing);
+
+      // No exception is thrown.
+      expect(tester.takeException(), isNull);
+  });
+
 }
 
 class _GeometryListener extends StatefulWidget {