How to reproduce:
1. Open any project with Cinemachine installed
2. Add a "Cinemachine Camera" Component to a GameObject
3. In the Component, select an Extention in the "Add Extention" field
4. In the same Component, in the "Add Extention" field select "(select)"
5. Observe the Console

Actual result: Null value is passed to the field, error in Console displayed
Expected result: By clicking the option, no value should be passed

Reproducible with: 3.0.0-pre.7

Error message:
"ArgumentException: Type cannot be null.
Unity.Cinemachine.Editor.CmCameraInspectorUtility+<>c__DisplayClass5_0.<AddExtensionsDropdown>b__0 (UnityEngine.UIElements.ChangeEvent`1[T] evt) (at ./Library/PackageCache/com.unity.cinemachine@3.0.0-pre.7/Editor/Utility/CmCameraInspectorUtility.cs:252)
UnityEngine.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.PropagationPhase propagationPhase) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.EventCallbackRegistry.InvokeCallbacks (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.PropagationPhase propagationPhase) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.CallbackEventHandler.HandleEvent (UnityEngine.UIElements.EventBase evt) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.CallbackEventHandler.HandleEventAtCurrentTargetAndPhase (UnityEngine.UIElements.EventBase evt) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.CallbackEventHandler.UnityEngine.UIElements.IEventHandler.HandleEvent (UnityEngine.UIElements.EventBase evt) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.EventDispatchUtilities.HandleEventAcrossPropagationPath (UnityEngine.UIElements.EventBase evt) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.EventDispatchUtilities.PropagateEvent (UnityEngine.UIElements.EventBase evt) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.DefaultDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.EventDispatcher.ApplyDispatchingStrategies (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, System.Boolean imguiEventIsInitiallyUsed) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.VisualElement.SendEvent (UnityEngine.UIElements.EventBase e) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.BaseField`1[TValueType].set_value (TValueType value) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.PopupField`1[T].set_value (T value) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.PopupField`1[T].ChangeValueFromMenu (T menuItem) (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEngine.UIElements.PopupField`1+<>c__DisplayClass26_0[T].<AddMenuItems>b__0 () (at <15230963ad214ef9972d854a56d0846d>:0)
UnityEditor.GenericMenu.CatchMenu (System.Object userData, System.String[] options, System.Int32 selected) (at <d6d088ded9754d9790b4a803014a22a0>:0)"