kOS V1.2.1.0
KSP 1.11.0.3045 (w/ Making History & Breaking Ground)

Steps to reproduce:

1. In VAB or SPH build a 3 part craft consisting of an OKTO2 core, a Z-200 battery and a CompoMax Radial Tubeless kOS processor. Align it vertically, so it won't start rolling when launched (SPH).
2. Launch it.
3. Open kOS terminal.
4. Switch SAS to on, either via console or via navball.
5. Hide navball via GUI.
6. Enter and execute set sasmode to "normal".

Result:
Error message: Object reference not set to an instance of an object
Expected result:
No message, activation of Normal Mode for SAS.

7. Unhide navball.
8. Enter and execute set sasmode to "normal".

Result: Expected result, no message, activation of Normal Mode for SAS.

Remarks:
Before testing, make sure the probe core supports the SAS mode to switch to, in the situation you test it in.
The problem is not specific to the the mentioned above 3 parts, it occurs with other SAS modes, probe cores and kOS processors, too.

This one was pretty surprising when it ruined a video recording of an automated space plane flight, where I wanted to reduce screen clutter by hiding UI elements. After about an hour of recording (autom. launch, orbit, deorbit, land), it made the "bleepblop" sound, and the plane went for an uncontrolled lithobraking. 😁