When a group model is created within AECOsim, it doesn't always show up in the model.
To avoid getting the blank screen when opening the group model you have to make sure the extraction preview (for any extraction) that usually loads up on View 8 is closed. This applies both when opening the group model from the extraction wizard or otherwise. Even in general switching between models or calculating the extraction is not advisable with the extraction preview open in view 8. Once you get the blank screen the only option seems to close and re-open the model.