final var prs = pullRequestRepository.findBySource_Id(id);
if (!prs.isEmpty()) {
    final var pr = prs.get(0);
    throw new BranchHasPullRequestException(
            pr.getId(), pr.getPrKey()
    );
}

public class BranchHasPullRequestException extends RuntimeException {
    private final Long prId;
    private final String prKey;
    // constructor, getters
}

@ExceptionHandler(BranchHasPullRequestException.class)
public ResponseEntity<Map<String, Object>> handleBranchHasPr(
        BranchHasPullRequestException ex) {
    return ResponseEntity.status(HttpStatus.CONFLICT).body(Map.of(
            "message", "Cannot delete branch — it has an associated pull request.",
            "prKey", ex.getPrKey(),
            "prId", ex.getPrId()
    ));
}

case 409: {
    const data = err.response?.data;
    const prKey = data?.prKey;
    const prId = data?.prId;
    const error = new Error(data?.message || "Cannot delete branch.");
    (error as any).prId = prId;
    (error as any).prKey = prKey;
    throw error;
}

} catch (err: unknown) {
    const e = err as any;
    if (e.prId) {
        toast.error(
            <span>
                Cannot delete branch —
                <a href={`/app/projects/${projectId}/pullrequests/${e.prId}`}>
                    View {e.prKey} →
                </a>
            </span>
        );
    } else {
        toast.error(e.message || "Failed to delete branch");
    }
}