Fix a floating snapping SliverAppBar crash (#26101)

packages/flutter/lib/src/rendering/sliver_persistent_header.dart

@@ -393,6 +393,7 @@ abstract class RenderSliverFloatingPersistentHeader extends RenderSliverPersiste
       return;
     if (value == null) {
       _controller?.dispose();
+      _controller = null;
     } else {
       if (_snapConfiguration != null && value.vsync != _snapConfiguration.vsync)
         _controller?.resync(value.vsync);

packages/flutter/test/material/app_bar_test.dart

@@ -1422,4 +1422,67 @@ void main() {
       statusBarIconBrightness: Brightness.dark,
     ));
   });
+
+  testWidgets('Changing SliverAppBar snap from true to false', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/17598
+
+    const double appBarHeight = 256.0;
+    bool snap = true;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return Scaffold(
+              body: CustomScrollView(
+                slivers: <Widget>[
+                  SliverAppBar(
+                    expandedHeight: appBarHeight,
+                    pinned: false,
+                    floating: true,
+                    snap: snap,
+                    actions: <Widget>[
+                      FlatButton(
+                        child: const Text('snap=false'),
+                        onPressed: () {
+                          setState(() {
+                            snap = false;
+                          });
+                        },
+                      ),
+                    ],
+                    flexibleSpace: FlexibleSpaceBar(
+                      background: Container(
+                        height: appBarHeight,
+                        color: Colors.orange,
+                      ),
+                    ),
+                  ),
+                  SliverList(
+                    delegate: SliverChildListDelegate(
+                      <Widget>[
+                        Container(height: 1200.0, color: Colors.teal),
+                      ],
+                    ),
+                  ),
+                ],
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    TestGesture gesture = await tester.startGesture(const Offset(50.0, 400.0));
+    await gesture.moveBy(const Offset(0.0, -100.0));
+    await gesture.up();
+
+    await tester.tap(find.text('snap=false'));
+    await tester.pumpAndSettle();
+
+    gesture = await tester.startGesture(const Offset(50.0, 400.0));
+    await gesture.moveBy(const Offset(0.0, -100.0));
+    await gesture.up();
+    await tester.pump();
+  });
 }