[qt] Avoid potential null pointer dereference in TransactionView::exportClicked()
Summary:
Backport of Core PR 10673
https://github.com/bitcoin/bitcoin/pull/10673/commits/fd9599b1358a314b073a9ca0a68ca8037915d91d
Test Plan:
make check
bitcoin-qt # sanity checks on export transaction history
Reviewers: deadalnix, Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D2875