Operating system: MacOS 10.13.6
Graphics card: AMD Radeon R9 M370X
Broken: 2.80, 266b1e2cdbc1, 2018-12-29
Short description of error
No driver variables seem to show their values while editing, even after hitting the update button. Driver final value does not show a value either. If the expression is invalid there is no indication of an error; it just says, "slow python expression" even if Python would not evaluate the code.
As an aside, changing the expression outputs for rotation channels to degrees should also be considered a bug. Please change back to radians! :)
Exact steps for others to reproduce the error
Attached a .blend. In Armature BodyArm, there is a driver on the z-axis rotation for bone calf.L.
On mch.leg.L there are custom properties. If you change tester the expression seems to work but none of the values change.
If you change the expression to something absurd like "bunnies", there is no error printed at all.