### 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.
GDScriptParser::IdentifierNode *identifier = nullptr;
	if (p_node->type == GDScriptParser::Node::IDENTIFIER) {
		identifier = static_cast<GDScriptParser::IdentifierNode *>(p_node);
	} else if (p_node->type == GDScriptParser::Node::SUBSCRIPT) {
		GDScriptParser::SubscriptNode *subscript = static_cast<GDScriptParser::SubscriptNode *>(p_node);
		if (subscript->is_attribute) {
			identifier = subscript->attribute;
		}
	}
	if (identifier == nullptr) {
		return;
	}
	GDScriptParser::Node *source = nullptr;
	switch (identifier->source) {
		case GDScriptParser::IdentifierNode::MEMBER_VARIABLE: {
			source = identifier->variable_source;
		} break;
		case GDScriptParser::IdentifierNode::FUNCTION_PARAMETER: {
			source = identifier->parameter_source;
		} break;
		case GDScriptParser::IdentifierNode::LOCAL_VARIABLE: {
			source = identifier->variable_source;
		} break;
		case GDScriptParser::IdentifierNode::LOCAL_ITERATOR: {
			source = identifier->bind_source;
		} break;
		default:
			break;