fix nullable issue

Samples/UIWidgetsSamples_2019_4/Assets/Editor/Tests/Stream/TestMain.cs

                     Debug.Log("val = " + val);
                 });
             }
+
+            /**
+             * Test StreamController
+             */
+            private void test11()
+            {
+                StreamController<float> controller = StreamController<float>.create();
+                Stream<float> stream = controller.stream;
+
+                var value = 1f;
+                var timer = Timer.periodic(new TimeSpan(0, 0, 0, 1), (v) =>
+                {
+                    value = value * 1.2f;
+                    controller.add(value);
+                    return null;
+                });
+                
+                stream.listen((val) =>
+                {
+                    if (val >= 2)
+                    {
+                        timer.cancel();
+                    }
+                    Debug.Log("value = " + val);
+                });
+            }
-                test10();
+                test11();
                 return new Container();
             }
         }

com.unity.uiwidgets/Runtime/async/stream_controller.cs

         public static StreamController<T> broadcast(
             Action onListen = null, Action onCancel = null, bool sync = false) {
             return sync
-                ? (StreamController<T>) new _SyncBroadcastStreamController<T>(() => onListen(), onCancel)
-                : new _AsyncBroadcastStreamController<T>(() => onListen(), () => {
-                    onCancel();
+                ? (StreamController<T>) new _SyncBroadcastStreamController<T>(() => onListen?.Invoke(), onCancel)
+                : new _AsyncBroadcastStreamController<T>(() => onListen?.Invoke(), () => {
+                    onCancel?.Invoke();
                     return Future._nullFuture;
                 });
         }
             }
 
             subscription._setPendingEvents(pendingEvents);
-            subscription._guardCallback(() => { _stream._runGuarded(() => onListen()); });
+            subscription._guardCallback(() => { _stream._runGuarded(() => onListen?.Invoke()); });
 
             return subscription;
         }
                 addState.pause();
             }
 
-            _stream._runGuarded(() => onPause());
+            _stream._runGuarded(() => onPause?.Invoke());
         }
 
         public override void _recordResume(StreamSubscription<T> subscription) {
             }
 
-            _stream._runGuarded(() => onResume());
+            _stream._runGuarded(() => onResume?.Invoke());
         }
     }