NullReferenceException error is thrown on assigning Font Atlas texture to None.

Steps to repro:
1. Create a new Project.
2. Windows > Package Manager > TextMeshPro(3.00-preview.11) > Update.
3. GameObject > 3D Object > Text - TextMeshPro.
4. Debug Settings > Font Atlas > Select texture to None.

Actual Result:
NullReferenceException error is thrown in the console.

Expected Result:
No error in the console.

Reproducible in:
2020.2.0a11, 2020.1.0b8, 2019.3.0f3.

Tested on TMP Package Version: 3.0.0-preview.11, 3.0.0-preview.1.

Environment:
Occurring on Windows 10 & Mac 10.15.

Stacktrace:
NullReferenceException: Object reference not set to an instance of an object
TMPro.TextMeshPro.ON_MATERIAL_PROPERTY_CHANGED (System.Boolean isChanged, UnityEngine.Material mat) (at library/packagecache/com.unity.textmeshpro@3.0.0-preview.11/scripts/runtime/tmpro_private.cs:335)
TMPro.FastAction`2[A,B].Call (A a, B b) (at library/packagecache/com.unity.textmeshpro@3.0.0-preview.11/scripts/runtime/fastaction.cs:109)
TMPro.TMPro_EventManager.ON_MATERIAL_PROPERTY_CHANGED (System.Boolean isChanged, UnityEngine.Material mat) (at library/packagecache/com.unity.textmeshpro@3.0.0-preview.11/scripts/runtime/tmpro_eventmanager.cs:57)
TMPro.EditorUtilities.TMP_BaseShaderGUI.OnGUI (UnityEditor.MaterialEditor materialEditor, UnityEditor.MaterialProperty[] properties) (at library/packagecache/com.unity.textmeshpro@3.0.0-preview.11/scripts/editor/tmp_baseshadergui.cs:166)
UnityEditor.MaterialEditor.PropertiesGUI () (at <89a088b11b1f446fb543c196a8473632>:0)
UnityEditor.MaterialEditor.OnInspectorGUI () (at <89a088b11b1f446fb543c196a8473632>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <89a088b11b1f446fb543c196a8473632>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)