Fixed DragTarget calling setState after dispose. (#6150)

DragTarget would call setState after it was disposed in didLeave
and didDrop. Fixes #6128.

packages/flutter/lib/src/widgets/drag_target.dart

@@ -316,6 +316,8 @@ class _DragTargetState<T> extends State<DragTarget<T>> {
 
   void didLeave(dynamic data) {
     assert(_candidateData.contains(data) || _rejectedData.contains(data));
+    if (!mounted)
+      return;
     setState(() {
       _candidateData.remove(data);
       _rejectedData.remove(data);
@@ -324,6 +326,8 @@ class _DragTargetState<T> extends State<DragTarget<T>> {
 
   void didDrop(dynamic data) {
     assert(_candidateData.contains(data));
+    if (!mounted)
+      return;
     setState(() {
       _candidateData.remove(data);
     });

packages/flutter/test/widget/draggable_test.dart

@@ -913,6 +913,7 @@ void main() {
     expect(didTap, isFalse);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/6128.
   testWidgets('Draggable plays nice with onTap', (WidgetTester tester) async {
     await tester.pumpWidget(new Overlay(
       initialEntries: <OverlayEntry>[
@@ -944,6 +945,65 @@ void main() {
     await firstGesture.moveBy(new Offset(100.0, 0.0));
     await secondGesture.up();
   });
+
+  testWidgets('DragTarget does not set state when remove from the tree', (WidgetTester tester) async {
+    List<String> events = <String>[];
+    Point firstLocation, secondLocation;
+
+    await tester.pumpWidget(new MaterialApp(
+      home: new Column(
+        children: <Widget>[
+          new Draggable<int>(
+            data: 1,
+            child: new Text('Source'),
+            feedback: new Text('Dragging')
+          ),
+          new DragTarget<int>(
+            builder: (BuildContext context, List<int> data, List<dynamic> rejects) {
+              return new Text('Target');
+            },
+            onAccept: (int data) {
+              events.add('drop');
+            }
+          ),
+        ]
+      )
+    ));
+
+    expect(events, isEmpty);
+    expect(find.text('Source'), findsOneWidget);
+    expect(find.text('Target'), findsOneWidget);
+
+    expect(events, isEmpty);
+    await tester.tap(find.text('Source'));
+    expect(events, isEmpty);
+
+    firstLocation = tester.getCenter(find.text('Source'));
+    TestGesture gesture = await tester.startGesture(firstLocation, pointer: 7);
+    await tester.pump();
+
+    await tester.pump(const Duration(seconds: 20));
+
+    secondLocation = tester.getCenter(find.text('Target'));
+    await gesture.moveTo(secondLocation);
+    await tester.pump();
+
+    await tester.pumpWidget(new MaterialApp(
+      home: new Column(
+        children: <Widget>[
+          new Draggable<int>(
+            data: 1,
+            child: new Text('Source'),
+            feedback: new Text('Dragging')
+          ),
+        ]
+      )
+    ));
+
+    expect(events, isEmpty);
+    await gesture.up();
+    await tester.pump();
+  });
 }
 
 class DragTargetData { }