gui: Set progressDialog to nullptr

Summary:
The progress dialog pointer must be set to nullptr after deletion,
else it can cause a crash is showProgress is called again with n>0.

Backport of Core PR16135

Test Plan:
ninja && ninja check

Run ./bitcoin-qt Menu help -> debug -> console
In the console, run:

rescanblockchain
dumpwallet /tmp/wallet
importwallet /tmp/wallet

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7799