Photomap add: If submission fails, going back results in a blank map

[mvl22]

I have now seen this on more than one device.

Reproduce case:

1. Go to Add photo
2. Take a photo
3. Do not add a caption (so that the API submission then fails)
4. Click Next
5. Set e.g. problem, cycle parking, or whatever
6. Map is shown. Click to set a map location if the photo could not be geolocated.
7. Click Upload.
8. API returns an error and therefore an error message shown. Click OK to dismiss it.
9. Screen returns to the map, but the map is now blank.
10. Moving or pinching the map has no effect - the tiles will not show.