message: Reactor: Error for invalid reference in URL
diff: import { useCallback, useMemo, useEffect } from 'preact/hooks';
  import { IntlProvider, Text, MarkupText } from 'preact-i18n';
  +import t from '../../Utility/i18n';
  import { Radio } from '../Radio';
  import { SvaFinder } from '../SvaFinder';
  import { mailto_handlers } from '../MailtoDropdown';
  @@ -56,13 +57,18 @@ const _Reactor = ({ reference }: ReactorProps) => {
  useEffect(() => {
  resetRequestToDefault({ advanceBatch: false, type: 'custom', reference });
  - const requestMessage = getGeneratedMessage(proceedingsState.proceedings[reference], 'request');
  + const proceeding = proceedingsState.proceedings[reference];
  + if (!proceeding)
  + throw new ErrorException(
  + 'Tried to use reactor with non-existent proceeding.',
  + { reference },
  + t('error-invalid-reference', 'reactor')
  + );
  - // TODO: Do we want to attach the proceeding (cf. `base.ts`)?
  + const requestMessage = getGeneratedMessage(proceeding, 'request');
  if (!requestMessage)
  - throw new ErrorException('Tried to use reactor with proceeding without initial request.', {
  - proceeding: proceedingsState.proceedings[reference],
  - });
  + // TODO: Do we want to attach the proceeding (cf. `base.ts`)?
  + throw new ErrorException('Tried to use reactor with proceeding without initial request.', { proceeding });
  if (requestMessage.slug) setCompanyBySlug(requestMessage.slug);
  else {
  