Crash on start up, bad assertion
The below error occurs when trying to start gnome-boxes after creating some VMs and then removing the installation media from their respective configs. I think it's a simple check that the VM has a source media, but when there is no longer a need for the source media and it's been removed then this check should be skipped. This occurred on Fedora 30 with two Fedora 30 VMs/Boxes.
$ gnome-boxes --version
3.32.1
$ gnome-boxes
(gnome-boxes:4792): Gtk-WARNING **: 09:41:43.240: GtkFlowBox with a model will ignore sort and filter functions
(gnome-boxes:4792): Gtk-WARNING **: 09:41:43.241: GtkListBox with a model will ignore sort and filter functions
(gnome-boxes:4792): Boxes-WARNING **: 09:41:44.035: libvirt-broker.vala:66: Failed to update domain 'fedora30-wor-2': Failed to set domain configuration: XML error: Invalid PCI address 0000:04:00.0. slot must be >= 1
(gnome-boxes:4792): Boxes-CRITICAL **: 09:41:44.035: boxes_vm_importer_get_source_media: assertion 'self != NULL' failed
Segmentation fault (core dumped)