No UTRs detected

I see the following warning when using geom_transcript, despite my GTF file having UTR features

```
Warning message in ggplot_add.transcript(object, p, objectname):
“No UTR detected in provided GTF!”
```

Warning persists even if I change the UTR feature types from e.g. five_prime_utr or three_prime_utr to utr or UTR.

I presume this is causing coverage in the UTRs not to show.