iOS 13-beta KVC闪退

在设置UITextField的Placeholder也就是占位文本的颜色和字体时使用了KVC的方法：
修改前:

[_textField setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];
[_textField setValue:[UIFont systemFontOfSize:14] forKeyPath:@"_placeholderLabel.font"];

修改后:

_textField.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"姓名" attributes:@{NSFontAttributeName:[UIFont systemFontOfSize:14],NSForegroundColorAttributeName:[UIColor redColor]}];