title: About NSArray nil crash error
body: <p>How to avoid this crash error!Crash in main thread Log is </p>
  
  <pre><code>ProcessArray:()
  ProcessArray == 0
  -[NSNull length]: unrecognized selector sent to instance 0x3c4e1090
  </code></pre>
  
  <p>my code:</p>
  
  <pre><code>ProcessArray = [EventSheetDetailArray  valueForKey:@"Process"];
  NSLog(@"ProcessArray:%@",ProcessArray);
  
  if (ProcessArray.count &gt; 0 ) {
      NSLog(@"ProcessArray != 0");
      [self ProcessJSONDateFormat];
  }else{
      NSLog(@"ProcessArray == 0");
  }
  </code></pre>
  
  <p>I change judge below!</p>
  
  <pre><code>id value = [EventSheetDetailArray valueForKey:@"Process"];
  if ( ![[NSNull null] isEqual:value] )
  {
      ProcessArray = value;
      NSLog(@"1111111");
  }else{
      NSLog(@"2222222");
  }
  </code></pre>
  
  <p>But always run NSLog(@"1111111") this line!
  Regardless of whether any value!</p>