title: Expression Validator gives "Unable to get a property on a non-object"
body: <p>can anybody figure out why this:</p>
  
  <pre><code>    $builder
          -&gt;add('networkLoIp', IntegerType::class, array(
              'constraints' =&gt; array(
                  new A\NotBlank(),
                  new A\Expression(array(
                      'expression' =&gt; 'value &lt;= this.getNetworkHiIp()'
                  ))
              )
          ))
          -&gt;add('networkHiIp', IntegerType::class, array(
              'constraints' =&gt; array(
                  new A\NotBlank()
              )
          ))
          -&gt;setMethod('post')
          ;
  </code></pre>
  
  <p>gives an error like this: <b>"Unable to get a property on a non-object"</b> ?</p>
  
  <p>When I dump data after submit I can see values I've put inside the form.</p>
  
  <p><b>EDIT</b></p>
  
  <p>A very similar error I get after moving expression to options, i.e.</p>
  
  <pre><code>public function configureOptions(OptionsResolver $optionsResolver)
  {
      $optionsResolver-&gt;setDefaults(array(
          'constraints' =&gt; array(
           new A\Expression(array(
              'expression' =&gt; "value['networkLoIp'] &lt;= value['networkHiIp']"
          ))
          )
      ));
  }
  </code></pre>
  
  <p><b>"Unable to get an item on a non-array."</b></p>