如何解决这个错误？ Xcode 7.2

Question

嗨，我正在尝试调整uipasteboard的大小但是我一直收到这个错误。有没有办法解决这个问题？

这是我正在使用的代码：

- (NSImage *)imageResize:(NSImage*)anImage newSize:(NSSize)newSize {
    NSImage *sourceImage = anImage;
    [sourceImage setScalesWhenResized:YES];

    // Report an error if the source isn't a valid image
    if (![sourceImage isValid]){
        NSLog(@"Invalid Image");
    } else {
        NSImage *smallImage = [[NSImage alloc] initWithSize: newSize];
        [smallImage lockFocus];
        [sourceImage setSize: newSize];
        [[NSGraphicsContext currentContext] setImageInterpolation:NSImageInterpolationHigh];
        [sourceImage drawAtPoint:NSZeroPoint fromRect:CGRectMake(0, 0, newSize.width, newSize.height) operation:NSCompositeCopy fraction:1.0];
        [smallImage unlockFocus];
        return smallImage;

    }

    return nil;
}