Reproduction Steps:

1. Open the attached project
2. Open Scene SampleScene
3. Dock the Project panel next to the Inspector (only one should be visible at any given time, but both should be initialized and available)
4. Select the State-Driven Camera while the Inspector is visible
5. Switch to the Project window
6. Create a new Animator Controller (RMB in Project view > Animator Controller)
7. Open it and create a new animation state
8. Click on various objects in the Hierarchy

Expected result: no errors

Actual result:

NullReferenceException: SerializedObject of SerializedProperty has been Disposed.
UnityEditor.SerializedProperty.get_intValue () (at /home/bokken/build/output/unity/unity/Editor/Mono/SerializedProperty.bindings.cs:1010)
Unity.Cinemachine.Editor.CinemachineStateDrivenCameraEditor+<>c__DisplayClass10_0.<CreateInspectorGUI>b__6 () (at ./Library/PackageCache/com.unity.cinemachine@3.0.0-pre.7/Editor/Editors/CinemachineStateDrivenCameraEditor.cs:63)
Unity.Cinemachine.Editor.InspectorUtility.<.cctor>g__OnUserDidSomething|1_0 () (at ./Library/PackageCache/com.unity.cinemachine@3.0.0-pre.7/Editor/Utility/InspectorUtility.cs:31)
UnityEditor.Selection.Internal_CallSelectionChanged () (at /home/bokken/build/output/unity/unity/Editor/Mono/Selection.cs:38)