[backport#16348] qt: Assert QMetaObject::invokeMethod result

Summary:
https://github.com/bitcoin/bitcoin/pull/16348/commits/64fee489448c62319e77941c30152084695b5a5d

Depends on D6181

Concludes backport of Core PR16348

Test Plan:

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

Move around UI make sure everything works

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6183