qt: Ensure that ModalOverlay is resized properly

Summary:

In certain circumstances the ModalOverlay widget is not sized properly:
With this PR the wallet frame looks ok

This is a backport of core#17968

Test Plan:
From core#17967

Run with -regtest, generate some blocks (200 in my case), then quit.
Remove rev00000.dat file to trigger an error.
Run with -regtest and in the dialog "Do you want to rebuild the block database now?" click "OK".

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9926