dotr (REPL) 'extension method on' highlight crash

## Minimized example

Try to type in REPL:

```Scala
extension as on (s: String) { def expand = ??? }
```

## Output

Right after typing 'on':
```scala
extension as on
Trying to highlight erroneous position [14..16]. Input size: 15
Trying to highlight erroneous position [14..16]. Input size: 15
...
```

## Expectation

No highlight error