### 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)

if (!array_prefix.is_empty()) {
			
			String str = p.name.trim_prefix(array_prefix);
			int to_char_index = 0;
			while (to_char_index < str.length()) {
				if (!is_digit(str[to_char_index])) {
					break;
				}
				to_char_index++;
			}
			if (to_char_index > 0) {
				array_index = str.left(to_char_index).to_int();
			} else {
				array_prefix = "";
			}
		}
		if (!array_prefix.is_empty()) {
			path = path.trim_prefix(array_prefix);
			int char_index = path.find("/");
			if (char_index >= 0) {
				path = path.right(-char_index - 1);
			} else {
				path = vformat(TTR("Element %s"), array_index);
			}
		} else {
			
			if (!subgroup.is_empty() && !subgroup_base.is_empty()) {
				if (path.begins_with(subgroup_base)) {
					path = path.trim_prefix(subgroup_base);
				} else if (subgroup_base.begins_with(path)) {