title: jsoncpp how to check if tag is null .isNull() throw assertion
body: <p>Im using jsoncpp , its great but when i need to check if json structure contains tag
  when i do it with : </p>
  
  <pre><code>UserRoot0["error"].isNull()
  </code></pre>
  
  <p>its throws me assert from json_value.cpp line 1025</p>
  
  <pre><code>JSON_ASSERT( type_ == nullValue  ||  type_ == objectValue );
  </code></pre>
  
  <p>i want to check if response im getting is from this type:</p>
  
  <pre><code>{
      "error" : {
          "message" : "Error validating application.",
          "type" : "OAuthException",
          "code" : 190
      }
  }
  </code></pre>