### Godot version

v4.0.beta17.mono.official [c40020513]

### System information

Window 11, Forward+, GTX 1060

### Issue description

Adding two theme variations in a row result to a  crash.

https://user-images.githubusercontent.com/93253123/217037249-1ce1d56d-beb6-4435-83ad-9f8a07fe0e96.mp4



### Steps to reproduce

1. Create a project.
2. Create a theme.
3. Double click to go in the theme editor.
4. Click on manage items buttons.
5. In the add type field type a name to add a type.
6. type another name to add a second type.
7. Crash.

### Minimal reproduction project

[ThemeVariationCrash.zip](https://github.com/godotengine/godot/files/10665863/ThemeVariationCrash.zip)

} else {
					subgroup = ""; 
				}
			}
			
			if (!group.is_empty() && !group_base.is_empty() && subgroup.is_empty()) {
				if (path.begins_with(group_base)) {
					path = path.trim_prefix(group_base);
				} else if (group_base.begins_with(path)) {
					
				} else {
					group = ""; 
					subgroup = "";
				}
			}
			
			if (!subgroup.is_empty()) {
				path = subgroup + "/" + path;
			}
			if (!group.is_empty()) {
				path = group + "/" + path;
			}
		}
		
		String name_override = (path.contains("/")) ? path.substr(path.rfind("/") + 1) : path;
		String feature_tag;
		{
			const int dot = name_override.find(".");
			if (dot != -1) {