修复部分场景下的crash问题

1. 当存在tab为flutter vc的情况，切换到首页后，tab的flutter vc其还是live的状态，当回到首页点击进入某个flutter vc，然后回到首页之后，这个时候进行前后台切换，tab的flutter vc会进行active的回调，进行surfaceUpdated操作，但是这个时候engine.viewController并不是这个tab的flutter vc，会产生crash
2. 当退出某个flutter vc的时候之前可能是为了时许问题不影响别的已经设置好的engine.viewController所以没有进行detatch操作，这样会有可能导致最后退出的lutter vc不能及时释放

ios/Classes/Engine/FLBFlutterEngine.m

@@ -30,7 +30,6 @@
 
 @interface FLBFlutterEngine()
 @property (nonatomic,strong) FlutterEngine *engine;
-@property (nonatomic,strong)  FLBFlutterViewContainer *dummy;
 @end
 
 @implementation FLBFlutterEngine
@@ -53,10 +52,6 @@
         }else{
             [_engine runWithEntrypoint:nil];
         }
-//        _dummy = [[FLBFlutterViewContainer alloc] initWithEngine:_engine
-//                                                          nibName:nil
-//                                                           bundle:nil];
-//        _dummy.name = kIgnoreMessageWithName;
     }
     
     return self;
@@ -108,9 +103,9 @@
 
 - (void)detach
 {
-    if(_engine.viewController != _dummy){
+    if(_engine.viewController != nil){
         [(FLBFlutterViewContainer *)_engine.viewController surfaceUpdated:NO];
-        _engine.viewController = _dummy;
+        _engine.viewController = nil;
     }
 }
 

ios/Classes/container/FLBFlutterViewContainer.m

@@ -38,6 +38,7 @@
 - (void)flushOngoingTouches;
 - (void)bridge_viewDidDisappear:(BOOL)animated;
 - (void)bridge_viewWillAppear:(BOOL)animated;
+- (void)surfaceUpdated:(BOOL)appeared;
 @end
 
 #pragma clang diagnostic push
@@ -223,6 +224,12 @@ static NSUInteger kInstanceCounter = 0;
     [FLUTTER_APP.flutterProvider detach];
 }
 
+- (void)surfaceUpdated:(BOOL)appeared {
+    if (self.engine && self.engine.viewController == self) {
+        [super surfaceUpdated:appeared];
+    }
+}
+
 #pragma mark - Life circle methods
 
 - (void)viewDidLayoutSubviews
@@ -288,6 +295,10 @@ static NSUInteger kInstanceCounter = 0;
                                                   params:_params
                                                 uniqueId:self.uniqueIDString];
     [super bridge_viewDidDisappear:animated];
+    
+    if (self.engine.viewController == self) {
+        [self detatchFlutterEngine];
+    }
 }
 
 - (void)installSplashScreenViewIfNecessary {