Description of problem:
Env:
- application theme set to Arc-Dark, night light enabled and active
Crash repro:
- start the app for the first time (two local-only calendars exist)
- switch to monthly view and add an event
- try to change the personal calendar color

Following attempts succeed.

Version-Release number of selected component:
gnome-calendar-3.30.1-1.fc29

Additional info:
reporter:       libreport-2.9.7
backtrace_rating: 4
cmdline:        /usr/bin/gnome-calendar --gapplication-service
crash_function: g_type_check_instance_cast
executable:     /usr/bin/gnome-calendar
journald_cursor: s=0328e63093a240c7910ac7ade6f6d745;i=6cfb3a;b=6e6f0e002f63450e8f6fa9dd34874863;m=919602138;t=57ecfd07fa753;x=e520cbc1df7bddab
kernel:         4.19.13-300.fc29.x86_64
rootdir:        /
runlevel:       N 5
type:           CCpp
uid:            1000

Truncated backtrace:
Thread no. 1 (2 frames)
 #0 g_type_check_instance_cast at gtype.c:4058
 #1 source_color_changed at ../src/gcal-source-dialog.c:573