Fix segfault on editor close up

[Chaosus]

Fix segfault in #23215

[groud]

I think using NOTIFICATION_THEME_CHANGED made sense, but it probably needs a check before running _update_display_mode().
Could you try something like that ? :

if (is_in_tree()) {
 _update_display_mode(true).
} 

[Chaosus]

True, its works ! How I didnt guess it myself...

[groud]

Awesome, thanks ! :)

[akien-mga]

Thanks!