Merged requested to merge verdre/gnome-shell:we-really-need-tests-for-this-shit into master
Since e06109c2 we keep old theme nodes valid during the emission of the "custom-stylesheets-changed" signal.
It turns out that we might still look up the file of a stylesheet using the files_by_stylesheet hashtable during the emission of that signal, causing a crash because the assertion in _st_theme_resolve_url() fails.
So fix that and remove the stylesheet entry from the files_by_stylesheet hashtable after emitting the "custom-stylesheets-changed" signal.
Fixes: #2757 (closed)