[backport#15462] gui: Fix async open wallet call order

Summary:
https://github.com/bitcoin/bitcoin/pull/15462/commits/a720a983015c9ef8cc814c16a5b9ef6379695817

Depends on D6175

Backport of Core PR15462

Test Plan:

ninja
./src/qt/bitcoin-qt -regtest

Open/close a wallet, UI behaves as expected

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6177