Fix crash after deleting a grouped connection

Fix gitlab.com/inkscape/inkscape/issues/205
10 jobs for master in 121 minutes and 15 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #231775690
appimage:linux

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passed #231775694
check-license-headers

00:01:44

passed #231775696
clangtidyfull:linux

01:58:55

passed #231775693
codeprotect

00:01:39

passed #231775692
allowed to fail
codequality

00:01:39

passed #231775689
inkscape:linux

00:13:42

passed #231775691
mac allowed to fail
inkscape:mac

00:12:52

passed #231775695
translations

00:02:37

 
  Test
passed #231775697
test:linux

00:02:18

 
  External
passed continuous-integration/appveyor/branch