Steps to repro:
1. Open the attached project and "SampleScene" scene;
2. In the Hierarchy select Reflection Probe gameobject;
3. In the Inspector press Bake button.

Expected result:
The reflection probe is successfully baked and the Scene/Game views are updated accordingly.

Actual result:
The reflection probe is not baked. The following warning is shown in the Console:

"OnDemand baking in progress...Can't read reflection probe lighting data asset until it's done.
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)"

See attached video.
Regression introduced in Unity 2021.2.0a3 and backported back to 2019.4.20f1.

Workaround: bake the Lighting through the Lighting window, this works as epxected.

Notes:
- Reproducible in Version 2021.2.0a3;
- Not reproducible in Version 2021.2.0a2, 2021.1.0b5, 2020.2.3f1;
- Reproducible on OSX 10.15.7;
- Reproducible only when initially baking probes via the probe Inspector;
- Reproducible only for the Baked reflection probe, not reproducible for the Custom reflection probe;
- Reproducible in URP and legacy RP; not reproducible in HDRP;
- Not reproducible after having successfully baked the probes via the Lighting window.