The root.panel may be null when the PanelRenderer UIReload callback is triggered

Steps to reproduce:

1. Open attached NullPanelDebug project
2. Open Scene.unity
3. Hit play

Actual results: The "PanelRenderer Sub" callback is triggered with a null panel

Expected results: Panel shouldn't be null

Reproducible with versions: 6000.6 6000.5

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): 

Notes:

• Also worth investigating: Enabling the Sub GameObject, then disabling, then pressing play seems to still render Sub even though it shouldn't.