Fix race in UiThreadUtil main handler lazy initialization

Author: locnp-active

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/bridge/UiThreadUtil.kt

@@ -15,7 +15,7 @@ import com.facebook.react.common.build.ReactBuildConfig
 public object UiThreadUtil {
 
   private val mainHandler: Handler by
-      lazy(LazyThreadSafetyMode.NONE) { Handler(Looper.getMainLooper()) }
+      lazy(LazyThreadSafetyMode.SYNCHRONIZED) { Handler(Looper.getMainLooper()) }
 
   /**
    * Returns the handler associated with the main (UI) thread.