code: public void setCellEditorValue(Object value) {
          datePicker.clear();
          if (value == null) {
              return;
          }
          if (value instanceof LocalDate) {
              LocalDate nativeValue = (LocalDate) value;
              datePicker.setDate(nativeValue);
          } else {
              String text = value.toString();
              String shorterText = InternalUtilities.safeSubstring(text, 0, 100);
              datePicker.setText(shorterText);
          }
      }
docs: setCellEditorValue, This sets the picker to an appropriate value for the supplied object. If
  the value is null, then picker will be cleared. If the value is a LocalDate instance, then
  the picker will be set to that value. All other types (including strings) will be read or
  converted to a string with a maximum length of the first 100 characters. The picker text will
  be set with the resulting string.