Steps to reproduce:
1. Open attached project and scene named "scene1".
2. Navigate to the "Assets" folder through "Project" window and select game object named "Test".
3. Press play button in the Editor.
3. Notice that console window does not output any error message.
4. Still in the play mode, in "Project" window select prefab named "New Prefab".
5. Notice that now console window continually outputs error message "NullReferenceException: Object reference not set to an instance of an object", because prefab has a "NetworkManagerHUD" component.
6. Again in "Project" window select prefab "Test" and no error messages are output until again selecting prefab "New Prefab".

GIF attached.

Expected result:
There should be no exceptions thrown only by selecting game object with "NetworkManagerHUD" component in the Editor.

Reproduced with:
5.4.0b13, 5.3.4p2, 5.3.4f1, 5.2.4f1.
With 5.1.0f3 error messages appear not constantly (only four messages per click on prefab "New Prefab").

Note:
Sometimes (not always) exiting Unity while getting these error messages causes crash: "Unity Editor has stopped working".