steps to reproduce:
- load two images
- open PointbasedRegistration Plugin
- select "Toggle point editing" button of first image
- add four points to first image with shift + left click
- unselect "Toggle point editing" button of first image
- select "Toggle point editing" button of second image
- toggle visibility of first datanode
-> all datanodes are invisible. Visibility of pointsets must be visible to add new points. So the user has to select and toggle the visibility of the pointset and image manually.