Color panel hits sigsegv in gcm_prefs_calib_apply_cb
The bug has downstream reports in Ubuntu and Fedora and is still getting reports with 3.30.2,
One example with a debug backtrace (2.28.2) is https://bugzilla.redhat.com/show_bug.cgi?id=1592087
#0 gcm_prefs_calib_apply_cb at ../panels/color/cc-color-panel.c:316
#5 on_assistant_apply at gtkassistant.c:1062
#6 _g_closure_invoke_va at gclosure.c:867
#9 gtk_button_do_release at gtkbutton.c:1843
#10 gtk_real_button_released at gtkbutton.c:1961
#11 _g_closure_invoke_va at gclosure.c:867
#14 multipress_released_cb at gtkbutton.c:666
#15 ffi_call_unix64 at ../src/x86/unix64.S:76
#16 ffi_call at ../src/x86/ffi64.c:525
#17 g_cclosure_marshal_generic_va at gclosure.c:1604