Pass a parameter to a REST service

I have the following groovy script

//-------------------------- Usuario ------------------------------------------------------
// create dialog
def dialog = com.eviware.soapui.support.UISupport.createConfigurationDialog( "Campo Usuario" );
dialog.addTextField( "usuario", "usuario" );
// init values and show
def map = new java.util.HashMap();
map.put( "usuario", "" );
if( dialog.show( map ))
{
// get target step
def step = testRunner.testCase.getTestStepByName( "Properties" );
// assign
step.setPropertyValue( "usuario", map.get( "usuario" ));
}
else testRunner.cancel( "Campo usuario no encontrado" );

but when I executed it .The script shows me the next message

"java.lang.NullPointerException:Cannot invoke method setPropertyValue() on null object error at line:13"

I would like to pass the "usuario" parameter to the followings services REST .How can implement it?

Thanks for all you help!!