[qt] Avoid potential null pointer dereference in TransactionView::exportClicked()
[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

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