Steps to reproduce:
1. Create empty game object and give it any child (another empty game object works)
2. Use keyboard arrows to expand the parent (left and right)
3. Try to expand the expanded parent again and notice that null reference exception if thrown

Expected outcome: Next/previous object becomes selected in hierarchy

Tested on Win10
Reproducible in: 5.3.0b4, 5.3.0b5, 5.3.0b6
Not reproducible in: 5.0.3f2, 5.1.4f1, 5.2.0p1, 5.2.1p4, 5.2.2p3, 5.3.0b3