Steps to repro:

1. Open attached project and 'ProgressiveLightmappingExample' scene;
2. In the Lighting window press 'Generate Lighting' button;
3. After some time (10-12 seconds) toggle 'Prioritize View' checkbox and untoggle it right away;
4. After 2-3 seconds repeat step 3 3-4 times.

Actual result:
Editor freezes and Unity process is unresponsible. See attached video and Editor.log.

Expected result:
Unity doesn't freeze and baking continues as expected.

Notes:
- Reproducible in 2019.2.0a4, 2019.1.0b2, 2018.3.3f1.
- Reproducible when baking on NVIDIA GeForce GTX 1070 and GeForce 640m with the latest NVIDIA drivers 417.71;
- Doesn't happen when baking with CPU PLM.