[backport#16231] gui: Fix open wallet menu initialization order

Summary:
The menu must be created before connecting to aboutToShow signal.

https://github.com/bitcoin/bitcoin/pull/16231/commits/5224be5a3354e1a22ea4d7f0e40aadfccdf67064

Depends on D6170

Backport of Core PR16231

Test Plan:

ninja

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6172