kivy: fix crash in show_private_key
kivy: fix crash in show_private_key

neocogent/electrum,cryptapus/electrum,digitalbitbox/electrum,wakiyamap/electrum-mona,fujicoin/electrum-fjc,fyookball/electrum,fujicoin/electrum-fjc,digitalbitbox/electrum,romanz/electrum,asfin/electrum,spesmilo/electrum,pooler/electrum-ltc,cryptapus/electrum,cryptapus/electrum,romanz/electrum,asfin/electrum,asfin/electrum,fyookball/electrum,wakiyamap/electrum-mona,digitalbitbox/electrum,kyuupichan/electrum,pooler/electrum-ltc,spesmilo/electrum,vialectrum/vialectrum,pooler/electrum-ltc,pooler/electrum-ltc,wakiyamap/electrum-mona,kyuupichan/electrum,fujicoin/electrum-fjc,digitalbitbox/electrum,neocogent/electrum,neocogent/electrum,kyuupichan/electrum,spesmilo/electrum,wakiyamap/electrum-mona,romanz/electrum,vialectrum/vialectrum,vialectrum/vialectrum,fyookball/electrum,spesmilo/electrum