fix: use right property attribute for shared_ptr in ObjC++ class

Author: CAMOBAP

packages/react-native-multinstance/ios/SandboxReactNativeDelegate.h

@@ -20,7 +20,7 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @interface SandboxReactNativeDelegate : RCTDefaultReactNativeFactoryDelegate
 
-@property (nonatomic, nullable) std::shared_ptr<const facebook::react::SandboxReactNativeViewEventEmitter> eventEmitter;
+@property (nonatomic, readwrite) std::shared_ptr<const facebook::react::SandboxReactNativeViewEventEmitter> eventEmitter;
 @property (nonatomic, assign) BOOL hasOnMessageHandler;
 @property (nonatomic, assign) BOOL hasOnErrorHandler;