After resetting the "Image Filter Stack", NullReferenceException is thrown on adding any properties, refer the attached video.

Steps to repro:
1. Create a new project
2. Windows > Package Manager > TerrainTools > Install
3. Project > right-click > Create > Image Filter Stack
4. Project > Image Filter Stack > Inspector > Reset
5. Project > Image Filter Stack > Add Any Property

Actual Result:
NullReferenceException is thrown on adding any properties in the Image Filter Stack after resetting

Expected Result:
No errors in the console and property added on the list

Reproducible in:
2020.2.0a14, 2020.2.0a13

Note: Reset functionality for Image Filter Stack is introduced in 2020.2.0a13

Environment:
Windows 10 & macOS 10.14

Stack Trace:
NullReferenceException: SerializedObject of SerializedProperty has been Disposed.
UnityEditor.SerializedProperty.get_arraySize () (at <d152ba57ae03448bbfbedbdc3373ab44>:0)
UnityEditor.Experimental.TerrainAPI.FilterStackView.InsertFilter (System.Int32 index, System.Type type, System.Boolean recordFullObject) (at library/packagecache/com.unity.terrain-tools@3.0.1-preview/editor/terraintools/filterstack/filterstackview.cs:229)
UnityEditor.Experimental.TerrainAPI.FilterStackView.AddFilter (System.Type type) (at library/packagecache/com.unity.terrain-tools@3.0.1-preview/editor/terraintools/filterstack/filterstackview.cs:204)
UnityEditor.Experimental.TerrainAPI.FilterStackView+<>c__DisplayClass15_0.<Init>b__0 () (at library/packagecache/com.unity.terrain-tools@3.0.1-preview/editor/terraintools/filterstack/filterstackview.cs:143)
UnityEditor.GenericMenu.CatchMenu (System.Object userData, System.String[] options, System.Int32 selected) (at <d152ba57ae03448bbfbedbdc3373ab44>:0)