protected void execChangedValue() throws ProcessingException {
        clearErrorStatus();
        if (getValue() != null) {
          Long key = getValue();
          if (getMappingTableField().getTable().findRowByKey(new Object[] { key }) == null) {
            getAddButton().setEnabled(true);
            getAddButton().requestFocus();
          } else {
            setErrorStatus("Mapping with ID " + key + " already in table");
            getAddButton().setEnabled(false);

            new ClientAsyncJob("SmartField clearer", ClientSession.get()) {
              @Override
              protected void runVoid(IProgressMonitor aMonitor) throws Throwable {
                Thread.sleep(50);
                setValue(null); // this doesn't work
                requestFocus();
              }
            }.schedule();
          }
        } else {
          getAddButton().setEnabled(false);
        }
      }

      @Override
      protected void execFilterLookupResult(LookupCall call, List<LookupRow> result) throws ProcessingException {
        ITable table = getMappingTableField().getTable();
        for (LookupRow row : result) {
          if (table.findRowByKey(new Object[] { row.getKey() }) != null) {
            row.setIconId(Icons.MarkedError);
            row.setEnabled(false);
          } else {
            row.setIconId(Icons.Empty);
          }
        }
      }