title: How I do check if the variable exist in Mule 3.2?
body: <p>How do check if the variable exist in Mule 3.2?
  How create or replace a variable?</p>
  
  <p>My Flow Exception is</p>
  
  <pre><code>    &lt;choice doc:name="Choice"&gt;
                  &lt;when expression="#[myVar==null]"&gt;
                      &lt;processor-chain&gt;                      
                             &lt;set-variable variableName="myVar" value="value1" doc:name="myVar"/&gt;
                      &lt;/processor-chain&gt;
                  &lt;/when&gt;
                  &lt;otherwise&gt;
                      &lt;processor-chain&gt;
                            &lt;set-variable variableName="myVar" value="#[variable:myVar]" doc:name="myVar"/&gt;                       
                      &lt;/processor-chain&gt;
                  &lt;/otherwise&gt;
              &lt;/choice&gt;
  </code></pre>
  
  <p>"myVar==null" does not work</p>