fix a BottomSheet nullable issue (#89362)

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

@@ -215,14 +215,22 @@ class _BottomSheetState extends State<BottomSheet> {
   }
 
   void _handleDragUpdate(DragUpdateDetails details) {
-    assert(widget.enableDrag);
+    assert(
+      widget.enableDrag && widget.animationController != null,
+      "'BottomSheet.animationController' can not be null when 'BottomSheet.enableDrag' is true. "
+      "Use 'BottomSheet.createAnimationController' to create one, or provide another AnimationController.",
+    );
     if (_dismissUnderway)
       return;
     widget.animationController!.value -= details.primaryDelta! / _childHeight;
   }
 
   void _handleDragEnd(DragEndDetails details) {
-    assert(widget.enableDrag);
+    assert(
+      widget.enableDrag && widget.animationController != null,
+      "'BottomSheet.animationController' can not be null when 'BottomSheet.enableDrag' is true. "
+      "Use 'BottomSheet.createAnimationController' to create one, or provide another AnimationController.",
+    );
     if (_dismissUnderway)
       return;
     bool isClosing = false;

packages/flutter/test/material/bottom_sheet_test.dart

@@ -23,6 +23,26 @@ void main() {
     expect(dyDelta1, isNot(moreOrLessEquals(dyDelta2, epsilon: 0.1)));
   }
 
+  testWidgets('Throw if enable drag without an animation controller', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/89168
+    await tester.pumpWidget(
+      MaterialApp(
+        home: BottomSheet(
+          onClosing: () {},
+          builder: (_) => Container(
+            height: 200,
+            color: Colors.red,
+            child: const Text('BottomSheet'),
+          ),
+        ),
+      ),
+    );
+
+    await tester.drag(find.text('BottomSheet'), const Offset(0.0, 150.0));
+
+    expect(tester.takeException(), isNotNull);
+  });
+
   testWidgets('Tapping on a modal BottomSheet should not dismiss it', (WidgetTester tester) async {
     late BuildContext savedContext;