### 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.
Variant initializer_value = make_expression_reduced_value(p_assignable->initializer, is_initializer_value_reduced);
			if (is_initializer_value_reduced) {
				p_assignable->initializer->is_constant = true;
				p_assignable->initializer->reduced_value = initializer_value;
			} else {
				push_error(vformat(R"(Assigned value for %s "%s" isn't a constant expression.)", p_kind, p_assignable->identifier->name), p_assignable->initializer);
			}
		}
		if (has_specified_type && p_assignable->initializer->is_constant) {
			update_const_expression_builtin_type(p_assignable->initializer, specified_type, "assign");
		}
		GDScriptParser::DataType initializer_type = p_assignable->initializer->get_datatype();
		if (p_assignable->infer_datatype) {
			if (!initializer_type.is_set() || initializer_type.has_no_type() || !initializer_type.is_hard_type()) {
				push_error(vformat(R"(Cannot infer the type of "%s" %s because the value doesn't have a set type.)", p_assignable->identifier->name, p_kind), p_assignable->initializer);
			} else if (initializer_type.kind == GDScriptParser::DataType::BUILTIN && initializer_type.builtin_type == Variant::NIL && !is_constant) {
				push_error(vformat(R"(Cannot infer the type of "%s" %s because the value is "null".)", p_assignable->identifier->name, p_kind), p_assignable->initializer);
			}
		} else {
			if (!initializer_type.is_set()) {
				push_error(vformat(R"(Could not resolve type for %s "%s".)", p_kind, p_assignable->identifier->name), p_assignable->initializer);
			}
		}
		if (!has_specified_type) {
			type = initializer_type;
			if (!type.is_set() || (type.is_hard_type() && type.kind == GDScriptParser::DataType::BUILTIN && type.builtin_type == Variant::NIL && !is_constant)) {
				type.kind = GDScriptParser::DataType::VARIANT;
			}
			if (p_assignable->infer_datatype || is_constant) {
				type.type_source = GDScriptParser::DataType::ANNOTATED_INFERRED;