Alot of the applets we haven’t yet created 19.04 variants. We will be creating the 19.04 builds just before the beta in February/March. We do this because the final vala and gcc compiler becomes available then. It saves us alot of unnecessary recompilation.
What you can do is edit the backports ppa in update -settings from disco to cosmic.
This will install the cosmic applets where the builds do not exist.
Alternatively you will have to reinstall 18.04 or 18.10