anchor.diff:      String receiverEmail = receiverEmailInput.value;
       String userName = senderNameInput.value;
       String userEmail = senderEmailInput.value;
  +    if(receiverName == null || receiverEmail == null || userName == null || userEmail == null) {
  +      querySelector('#form-error-alert').classes.remove('disappear');
  +      return new Future.value(null);
  +    }
  +      
       //determine massage type
       InputElement messageInput= querySelector('.tab-content .active input');
       String message;
  
positive.code:     switch(messageInput.type) {
        case 'text':
          message = messageInput.value;
          break;
        case 'file':
          isFile = true;
  
negative.code:           int pos = messageFile.name.indexOf('.');
            filename = 'HackMessage' + messageFile.name.substring(pos);
            print(filename);
          } else {
            querySelector('#form-error-alert').classes.remove('disappear');
            return new Future.value(null);
          }
          break;
      }
      return uploadMsg(userName, userEmail, receiverName, receiverEmail, isFile, message, messageFile, filename)
      .then((_) {
        window.location.href = '../authen_done';
      })
  