[Bug]: Exclude bug crashes page

### Your DoenetML

```xml
<selectFromSequence name="a" from="1" to="10" numToSelect="2" exclude="2$a[1]"/>
```

### What happened?

Excluding a self referenced value crashes doenet with no warnings/errors. 

### What version of DoenetML are you using?

0.7.21

### What browsers are you seeing the problem on?

Chrome