gala crashes with SIGSEGV in g_type_check_instance_is_fundamentally_a ()

[quequotion]

We've been having some trouble with gala lately in Archlinux

https://bbs.archlinux.org/viewtopic.php?id=152930&p=18

Gala seems to always crash.

`coredumpctl gdb` says:

Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007fd08fd3fc4d in g_type_check_instance_is_fundamentally_a () from /usr/lib/libgobject-2.0.so.0

Arch's glib2 is 2.42.1, a few releases ahead of Ubuntu, so I thought it might be an incompatibility there.

I noticed, however, that we're not the only ones having trouble with gala (bug 1017168).

I'm filing this as a separate bug because of the different debugger output.