If you apply all overrides to a prefab and then click inside the overrides popup, a NullReferenceException will occur.

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PrefabOverridesTreeView.RowGUI (UnityEditor.IMGUI.Controls.TreeView+RowGUIArgs args) (at C:/buildslave/unity/build/Editor/Mono/Prefabs/PrefabOverrides/PrefabOverridesTreeView.cs:396)

1. Install Unity
2. Open the attached project
3. Open SampleScene.scene
4. Select the Cube object in the Scene Hierarchy
5. Make changes to the Box Collider component in the Inspector
6. Select Overrides in the Inspector
7. Select the Box Collider component in the Overrides Popup
8. Select Apply > Apply to Prefab 'Cube'
9. Click on the area that says "No Overrides" in the Override Popup

Expected Result:
No exceptions occur.

Actual Result:
A NullReferenceException occurs.

Regression in 2019.1.0a9.

Tested on 2019.1.0a9, 2019.1.0a8