qt: Fix crash in files with malformed signatures

For signatures we need the formwidget, so even if they don't have the
Subtype Widget create one when asked for signatures in the document

Fixes KDE bug #433909
7 jobs for fix_kde_433909 in 7 minutes and 4 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #7850441
build

00:06:41

passed #7850446
build_android

00:02:35

passed #7850442
build_clang_libcpp

00:07:04

passed #7850445
build_clazy

00:06:10

passed #7850444
build_mingw64_fedora

00:04:04

passed #7850443
build_ubuntu_18_04

00:04:06

passed #7850440
clang_format

00:01:37