gui: Fix itemWalletAddress leak when not tree mode

Summary: This is a backport of Core PR18578

Test Plan:
ninja && src/qt/bitcoin-qt
In the Send tab, click the "Inputs" button, select "Tree mode".

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9097