Fix dropdown crash (#69211)

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

@@ -456,7 +456,9 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
   }
 
   void _dismiss() {
-    navigator?.removeRoute(this);
+    if (isActive) {
+      navigator?.removeRoute(this);
+    }
   }
 
   double getItemOffset(int index) {

packages/flutter/test/material/dropdown_test.dart

@@ -2615,4 +2615,52 @@ void main() {
     expect(value, equals('two'));
     expect(menuItemTapCounters, <int>[0, 2, 1, 0]);
   });
+
+  testWidgets('does not crash when option is selected without waiting for opening animation to complete', (WidgetTester tester) async {
+    // Regression test for b/171846624.
+
+    final List<String> options = <String>['first', 'second', 'third'];
+    String? value = options.first;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: StatefulBuilder(
+            builder: (BuildContext context, StateSetter setState) => DropdownButton<String>(
+              value: value,
+              items: options.map((String s) => DropdownMenuItem<String>(
+                value: s,
+                child: Text(s),
+              )).toList(),
+              onChanged: (String? v) {
+                setState(() {
+                  value = v;
+                });
+              },
+            ),
+          ),
+        ),
+      ),
+    );
+    expect(find.text('first').hitTestable(), findsOneWidget);
+    expect(find.text('second').hitTestable(), findsNothing);
+    expect(find.text('third').hitTestable(), findsNothing);
+
+    // Open dropdown.
+    await tester.tap(find.text('first').hitTestable());
+    await tester.pump();
+
+    expect(find.text('third').hitTestable(), findsOneWidget);
+    expect(find.text('first').hitTestable(), findsOneWidget);
+    expect(find.text('second').hitTestable(), findsOneWidget);
+
+    // Deliberately not waiting for opening animation to complete!
+
+    // Select an option in dropdown.
+    await tester.tap(find.text('third').hitTestable());
+    await tester.pump();
+    expect(find.text('third').hitTestable(), findsOneWidget);
+    expect(find.text('first').hitTestable(), findsNothing);
+    expect(find.text('second').hitTestable(), findsNothing);
+  });
 }