code: protected String safeGetPostbackPayload(MessageEnvelope envelope) {
  		if (envelope != null && envelope.getPostback() != null
  				&& envelope.getPostback().getPayload() != null) {
  			return envelope.getPostback().getPayload();
  		}
  		return "";
  	}
docs: Retrieves a text message from an envelope. It never returns null.
  
  @param envelope
  the message envelope.
  @return the message envelope if found or an empty String otherwise. It
  never returns null.