How to reproduce:
1. Open new Unity project
2. Go to Edit --> Project Settings --> Quality
3. Delete all the quality levels and try to delete the last one

Expected result: Quality level is not deleted, but neither errors are produced
Actual result: Quality level is not deleted as expected, but errors are produced

Note: The issue is also reproduced by deleting all the Quality levels, except the last one, adding a new level and trying to delete it. Results in errors being produced and the Quality level not being deleted

Reproducible with - 2017.1.0b2, 5.6.0p2, 5.6.0b1
Not reproducible with - 5.5.3f1, 5.4.5f1