GoogleMobileAds框架编译错误

Question

尝试为iOS远程编译Google Mobile Ads框架时，出现此错误：

:     In file included from D:\Program Files\Epic Games\UE_4.21\Engine\Intermediate\UnzippedFrameworks\ThirdParty\IOS\Google\GoogleMobileAds.embeddedframework\GoogleMobileAds.framework\Headers\GoogleMobileAds.h:38:
LogPlayLevel: Error:     D:\Program Files\Epic Games\UE_4.21\Engine\Intermediate\UnzippedFrameworks\ThirdParty\IOS\Google\GoogleMobileAds.embeddedframework\GoogleMobileAds.framework\Headers\GADMediaContent.h:21:1: error: no 'assign', 'retain', or 'copy' attribute is specified - 'assign' is assumed [-Werror,-Wobjc-property-no-attribute]
LogPlayLevel:     @property(nonatomic, nullable) UIImage *mainImage;
LogPlayLevel:     ^

这样的文件非常简单：

//
//  GADMediaContent.h
//  Google Mobile Ads SDK
//
//  Copyright 2019 Google Inc. All rights reserved.
//

#import <UIKit/UIKit.h>

NS_ASSUME_NONNULL_BEGIN

/// Provides media content information. Interact with instances of this 
class on the main queue
/// only.
 @interface GADMediaContent : NSObject

/// Media content aspect ratio (width/height). The value is 0 when there's 
no media content or the
/// media content aspect ratio is unknown.
@property(nonatomic, readonly) CGFloat aspectRatio;

/// The main image to be displayed when the media content doesn't contain video.
@property(nonatomic, nullable) UIImage *mainImage;

@end

NS_ASSUME_NONNULL_END

所以这不是我的代码，来自Google。我正在从Windows计算机上使用Unreal Engine iOS远程编译