Can't display more than 1 slice representation of a shared dataset at a time since v0.0.7 #48

In v0.0.6, you could display multiple slice representations, e.g. in the FLORIS Flow app:

However, from v0.0.7 onwards, we are unable to add more than 1 slice representation:

The code to the app can be found here. Here's a playground demo of the app running on the latest vtk version.