NullReferenceException error is thrown on Selecting Cancel button from the "Create Impulse Channel Definition asset" popup while editing the "Impulse Channel" in "Cinemachine Impulse source (Script)" Component, refer attached video.

Steps to repro:
1. Create a new Project.
2. Windows > Package Manager > Install Cinemachine package 2.5.0
3. Hierarchy > Create Empty GameObject
4. Add " Cinemachine Impulse source (Script)" component in the Inspector
5. Impulse Channel > Edit > Create Impulse Channel Definition asset window > Cancel

Actual Result:
NullReferenceException error is thrown while editing Impluse Channel.

Expected Result:
Selecting Cancel while editing Impulse Channel shouldn't cause the editor to throw any error.

Reproducible in Package Version: 2.3.5, 2.4.0, 2.3.3, 2.2.0

Tested in Unity Version:
2020.1.0a20 , 2019.3.0f6, 2018.2.21f1, 2018.1.0a1

Environment:
Windows 10 & Mac 10.14.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
Cinemachine.Editor.CinemachineImpulseChannels.get_Instance () (at library/packagecache/com.unity.cinemachine@2.5.0/editor/impulse/cinemachineimpulsechannels.cs:55)
Cinemachine.Editor.CinemachineImpulseChannelPropertyDrawer.OnGUI (UnityEngine.Rect rect, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at library/packagecache/com.unity.cinemachine@2.5.0/editor/impulse/cinemachineimpulsechannelpropertydrawer.cs:45)
UnityEditor.PropertyDrawer.OnGUISafe (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.Rect visibleArea) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.EditorGUI.PropertyFieldInternal (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.EditorGUI.PropertyField (UnityEngine.Rect position, UnityEditor.SerializedProperty property, System.Boolean includeChildren) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.EditorGUI.PropertyField (UnityEngine.Rect position, UnityEditor.SerializedProperty property) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
Cinemachine.Editor.CinemachineImpulseDefinitionPropertyDrawer.OnGUI (UnityEngine.Rect rect, UnityEditor.SerializedProperty prop, UnityEngine.GUIContent label) (at library/packagecache/com.unity.cinemachine@2.5.0/editor/impulse/cinemachineimpulsedefinitionpropertydrawer.cs:83)
UnityEditor.PropertyDrawer.OnGUISafe (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.Rect visibleArea) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.PropertyHandler.OnGUILayout (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEditor.Editor.DrawPropertiesExcluding (UnityEditor.SerializedObject obj, System.String[] propertyToExclude) (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
Cinemachine.Editor.BaseEditor`1[T].DrawRemainingPropertiesInInspector () (at library/packagecache/com.unity.cinemachine@2.5.0/editor/utility/baseeditor.cs:78)
Cinemachine.Editor.CinemachineImpulseSourceEditor.OnInspectorGUI () (at library/packagecache/com.unity.cinemachine@2.5.0/editor/impulse/cinemachineimpulsesourceeditor.cs:16)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <bba5ae680bc14113ade3be5cc43cbed6>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)