Grouping Image data causes a crash

[alxvth]

Steps to reproduce:

1. Load an image
2. Load the same image again
3. Group the two images (explicitly the image data, not the points)

I suspect this to happen with all data types except points

Possible solution:

• Disallow grouping of all data types except points

[ThomasKroes]

There is a way to handle this with the existing architecture, we can talk later in more detail.