title: Google apps script method getDocumentProperties returns null
body: <p>I am using document properties in my google apps script to store the mysql database connection string and get the error TypeError: Cannot call method "setProperties" of null at line 2 not sure what the reason might be. </p>
  
  <pre><code>  dbmysqlconndev = PropertiesService.getDocumentProperties();
    dbmysqlconndev.setProperties({
     'mysqlUrl': 'jdbc:mysql://dbserverdev;databaseName=sample',
     'mysqlUser': 'username',
     'mysqlPassword': 'password'});
    var keys = dbmysqlconndev.getKeys();
    for (var i = 0; i &lt; keys.length; i++) {
         dbmysqlconndev.getProperty(keys[i]);
    }
  
  var mysqlurl = dbmysqlconndev.getProperty(keys[0]);
  var mysqlusr = dbmysqlconndev.getProperty(keys[1]);
  var mysqlpwd = dbmysqlconndev.getProperty(keys[2]);
  </code></pre>