To reproduce:
1) Open attached project and 'playingScene'
2) Build and run it
3) In the player, select "Host"
4) After it finishes loading, press 'Escape' to pause it and regain mouse control
5) While the game is still on, go to Editor and play the scene
6) Select 'Join'
7) After it finishes loading, press 'Escape' to pause it and regain mouse control
8) In Hierarchy, select 'normalPlanet'

Expected: Inspector shows the details of selected object
Actual: Editor crashes

This does not happen if Editor is Hosting while Player is Joining.

Reproduced in 5.5.0b5, 5.5.0f3, 5.6.0b3

Cannot test on 5.4 as the scene is broken upon loading the project