fix Qt crash

Summary:
introduced in D2322 backport

Didn't affect most people, only those who had custom fee selected (which required using Coin Control)

Workaround for affected users: change nCustomFeeRadio=1 to nCustomFeeRadio=0 in the ~/.config/BitcoinABC/<whatever>.conf file.

same fix was also required in core: https://github.com/bitcoin/bitcoin/pull/11332

Test Plan: it runs now!

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2559