Merge pull request react-native-maps#2501 from guoliang1206/master

[iOS] Fixed issue that app will crash after trigger  Marker onDragEnd

Author: alvelig

lib/ios/AirMaps/AIRMapManager.m

@@ -37,7 +37,9 @@ @interface AIRMapManager() <MKMapViewDelegate>
 
 @end
 
-@implementation AIRMapManager
+@implementation AIRMapManager{
+   BOOL _hasObserver;
+}
 
 RCT_EXPORT_MODULE()
 
@@ -704,11 +706,12 @@ - (void)mapView:(AIRMap *)mapView
         if (mapView.onMarkerDragEnd) mapView.onMarkerDragEnd(event);
         if (marker.onDragEnd) marker.onDragEnd(event);
 
-        [view removeObserver:self forKeyPath:@"center"];
+       if(_hasObserver) [view removeObserver:self forKeyPath:@"center"];
+        _hasObserver = NO;
     } else if (newState == MKAnnotationViewDragStateStarting) {
         // MapKit doesn't emit continuous drag events. To get around this, we are going to use KVO.
         [view addObserver:self forKeyPath:@"center" options:NSKeyValueObservingOptionNew context:&kDragCenterContext];
-
+        _hasObserver = YES;
         if (mapView.onMarkerDragStart) mapView.onMarkerDragStart(event);
         if (marker.onDragStart) marker.onDragStart(event);
     }