fix crash in onFeaturesDetected

Author: macrozone

ios/RCTARKit.m

@@ -451,10 +451,11 @@ - (void)session:(ARSession *)session didUpdateFrame:(ARFrame *)frame {
                                            }];
 
             }
-            
+            if(self.onFeaturesDetected) {
             self.onFeaturesDetected(@{
                                       @"featurePoints":featurePoints
                                       });
+            }
         });
     }
 }