qt: Fix regression in TransactionTableModel

Summary:
Since PR17993 a crash is possible on exit.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>

This is a backport of core-gui#8

Depends on D9254

Test Plan:
start bitcoin-qt
wait until sync
on main window: Menu -> File -> Quit

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9257