[Backport] gui: Fix start timer from non QThread

Summary:
This fixes a bug related to a QTimer that in some situation
can be started from outside a QThread.

Backport of Core PR17120

Test Plan: ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7568