### Godot version

4.1 rc3

### System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 30.0.15.1403) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

### Issue description

This™
![image](https://github.com/godotengine/godot/assets/2223172/cb33110f-a5b0-400a-951f-3fa425f05714)

It can still happen, but it's more difficult to trigger.

### Steps to reproduce

1. Create a new scene with a Control node
2. Add Theme to the Control
3. Open Theme Editor
4. Clear the `theme` property in Control
5. Save, with Theme Editor opened
6. Restart

### Minimal reproduction project

N/A
if (singleton->cmdline_export_mode) {
		print_line(p_task + ": begin: " + p_label + " steps: " + itos(p_steps));
	} else if (singleton->progress_dialog) {
		singleton->progress_dialog->add_task(p_task, p_label, p_steps, p_can_cancel);
	}
}
bool EditorNode::progress_task_step(const String &p_task, const String &p_state, int p_step, bool p_force_refresh) {
	if (singleton->cmdline_export_mode) {
		print_line("\t" + p_task + ": step " + itos(p_step) + ": " + p_state);
		return false;
	} else if (singleton->progress_dialog) {
		return singleton->progress_dialog->task_step(p_task, p_state, p_step, p_force_refresh);
	} else {
		return false;
	}
}
void EditorNode::progress_end_task(const String &p_task) {
	if (singleton->cmdline_export_mode) {
		print_line(p_task + ": end");
	} else if (singleton->progress_dialog) {
		singleton->progress_dialog->end_task(p_task);
	}
}
void EditorNode::progress_add_task_bg(const String &p_task, const String &p_label, int p_steps) {