Github上别人写的一个Category，叫做NullSafe ，在运行时操作，把这个讨厌的空值置为nil，而nil是安全的，可以向nil对象发送任何message而不会奔溃。这个category使用起来非常 方便，只要加入到了工程中就可以了，你其他的什么都不用做，对，就是这么简单。详细的请去Github上查看；
https://github.com/nicklockwood/NullSafe

#import <objc/runtime.h>
#import <Foundation/Foundation.h>


#ifndef NULLSAFE_ENABLED
#define NULLSAFE_ENABLED 1
#endif


#pragma clang diagnostic ignored "-Wgnu-conditional-omitted-operand"


@implementation NSNull (NullSafe)

#if NULLSAFE_ENABLED

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
    //look up method signature
    NSMethodSignature *signature = [super methodSignatureForSelector:selector];
    if (!signature)
    {
        for (Class someClass in @[
            [NSMutableArray class],
            [NSMutableDictionary class],
            [NSMutableString class],
            [NSNumber class],
            [NSDate class],
            [NSData class]
        ])
        {
            @try
            {
                if ([someClass instancesRespondToSelector:selector])
                {
                    signature = [someClass instanceMethodSignatureForSelector:selector];
                    break;
                }
            }
            @catch (__unused NSException *unused) {}
        }
    }
    return signature;
}

- (void)forwardInvocation:(NSInvocation *)invocation
{
    invocation.target = nil;
    [invocation invoke];
}

#endif

@end