System Information
Operating system: Linux-5.4.0-42-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: GeForce RTX 2070 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 440.100

Blender Version
Broken: version: 2.83.4, branch: master, commit date: 2020-08-05 06:00, hash: rBc113af82881a
Worked: (newest version of Blender that worked as expected)

Short description of error
when an object is used as cage for normal baking and next this object is removed, the cage object remains the same and it's not updated

Exact steps for others to reproduce the error
from the attached file:

Note that the "selected to active" and "cage" options are checked
press "bake" and the normal map is baked as usual (I assume that, under the hood, the active object is used as cage object, despite this is not explicit)
select the object named as "low_invalid" as object cage
hit bake again and you'll get an error, since the the active object and the cage object don't share the same topology
remove the "low_invalid" object from the cage object option (that should solve the problem, right?)
if you hit bake, you get the error message again
if you try to solve the problem by using the "low" object as the cage object, you get an empty normal map (not sure if this last part should work this way)

Tried in the 2.91 alpha and got the same results.