Bug report

Describe the bug

See https://forum.image.sc/t/tranform-annotations-not-working-in-qupath/115432

To Reproduce

Steps to reproduce the behavior:

1. Open an image
2. Create multiple annotations
3. Turn on selection mode, and use this to select all the annotations
4. Run Objects → Annotations → Transform annotations...
5. See the error

Expected behavior

Either a more informative error, or using selection mode ensures that a 'main' selection is also made.

Desktop (please complete the following information):

• OS: All
• QuPath Version: v0.6.0 (presumably earlier as well)

Additional context

I'm not sure if there are other commands that assume if getSelectedObjects() isn't empty, then getSelectedObject() is non-null.