Invalid CSS drop-shadow call crashes app

I'm finding crashes when messing with CSS drop-shadow in Workbench. When the syntax is slightly wrong, it crashes the whole app.

Here's a minimal reproduction:

1. Open the "Welcome" demo from the library (I think any other demo will do, but this is one I've checked)
2. Open the Style pane to edit CSS
3. Paste in the following CSS code:

   button {
     filter: drop-shadow(1px 0);
   }
   

4. Workbench crashes, repeatedly until I manually find and edit the CSS file it's using to remove the invalid drop-shadow() call

For context of the provided CSS snippet, it's one example I encountered while trying to type drop-shadow(1px 0px). It appears that Workbench pulled and tried to run the code as I was typing, before I made it to typing the "px" keys.

Once Workbench is in the crashing state (trying to load the invalid code), launching it from terminal prints the following:

$ flatpak run re.sonny.Workbench
(re.sonny.Workbench:5): Gjs-WARNING **: 14:16:41.982: Type GITypeInfo of property Gtk.IconTheme::search-path does not match return type GITypeInfo of getter get_search_path. Falling back to slow path
(re.sonny.Workbench:5): Gjs-WARNING **: 14:16:41.982: Type GITypeInfo of property Gtk.IconTheme::search-path does not match type GITypeInfo of first argument of setter set_search_path. Falling back to slow path
**
Gtk:ERROR:../gtk/gtkcsscolorvalue.c:2194:gtk_css_color_value_get_rgba: assertion failed: (color->type == COLOR_TYPE_COLOR)
Bail out! Gtk:ERROR:../gtk/gtkcsscolorvalue.c:2194:gtk_css_color_value_get_rgba: assertion failed: (color->type == COLOR_TYPE_COLOR)

Reproduced using Workbench 50.0, Flatpak 1.12.7, on Elementary OS 7.1.

This may include other CSS functions, I'm not sure, this is just the one I am aware of.

Start by reproducing the crash from the Welcome demo through the Style pane with the invalid drop-shadow() CSS shown in the issue. Trace how the Style pane applies CSS while text is being edited, then verify that invalid CSS no longer crashes Workbench and that valid CSS can still be entered.