### Godot version

Beta 17, master

### System information

Windows 11

### Issue description

Godot gets a hard crash while attempting to code-complete for member `const` of a class.

### Steps to reproduce

```GDScript
enum E { V1 }

const e := E
```

And then type a `.` after the last `E`, which triggers code hints/auto-completion for subscripts based on `E` and causes a hard crash.

Does not happen for `var e := E.V1`.

### Minimal reproduction project

See above.
} else if (callee_type == GDScriptParser::Node::IDENTIFIER) {
		base_type = parser->current_class->get_datatype();
		base_type.is_meta_type = false;
		is_self = true;
	} else if (callee_type == GDScriptParser::Node::SUBSCRIPT) {
		GDScriptParser::SubscriptNode *subscript = static_cast<GDScriptParser::SubscriptNode *>(p_call->callee);
		if (subscript->base == nullptr) {
			
			p_call->set_datatype(call_type);
			mark_node_unsafe(p_call);
			return;
		}
		if (!subscript->is_attribute) {
			
			
			p_call->set_datatype(call_type);
			mark_node_unsafe(p_call);
			return;
		}
		if (subscript->attribute == nullptr) {
			
			p_call->set_datatype(call_type);
			mark_node_unsafe(p_call);
			return;
		}
		GDScriptParser::IdentifierNode *base_id = nullptr;
		if (subscript->base->type == GDScriptParser::Node::IDENTIFIER) {
			base_id = static_cast<GDScriptParser::IdentifierNode *>(subscript->base);
		}
		if (base_id && GDScriptParser::get_builtin_type(base_id->name) < Variant::VARIANT_MAX) {