"Use G_ADD_PRIVATE" crashes on glib 2.56.2

Commit a4d1df87 causes zathura to crash at start:

**
GLib-GObject:ERROR:/var/tmp/portage/dev-libs/glib-2.56.2/work/glib-2.56.2/gobject/gobject.c:1807:g_object_new_internal: assertion failed: (nqueue
!= NULL)

glib 2.56.2 is the current version on "gentoo unstable". Ubuntu bionic beaver appears to have 2.56.1.

The crash appears on both zathura installed via package manager (configured to use git develop branch) and the one manually built from git develop branch (commit 8272e37f). The crash goes away if commit a4d1df87 is reverted.