Tutorial is non-responsive if the next hint is clicked at the last hint

[cswclui]

For multi-step hints, the Next Hint button can be clicked at the last hint (the button is greyed but it can be clicked). The Start Over and Run Code button becomes non-responsive after.

Example learnr tutorial:

# Test

```{r demo, exercise = TRUE}
1+1
```

```{r demo-hint-1, eval=FALSE}
hint1
```

```{r demo-hint-2, eval=FALSE}
hint2
```

[gadenbuie]

Thanks for the bug report! This should be fixed now 😄