Steps to reproduce:

1. Open attached project "font-features.zip"

2. Open scene "Scenes/SampleScene"

3. In hierarchy, select "Canvas > Text (TMP)" object

4. In inspector, on TextMeshPro - Text (UI) component, expand Extra Settings

5. Click on Font Features dropdown to open it

Expected results:  Dropdown opens with all available options

Actual results: Dropdown shows empy panel, throws NullReferenceException in console:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.MaskFieldDropDown.OnGUI (UnityEngine.Rect rect) (at /Users/bokken/build/output/unity/unity/Editor/Mono/Inspector/MaskFieldDropdown.cs:182)
UnityEditor.PopupWindow.OnGUI () (at /Users/bokken/build/output/unity/unity/Editor/Mono/GUI/PopupWindow.cs:122)
UnityEditor.HostView.OldOnGUI () (at /Users/bokken/build/output/unity/unity/Editor/Mono/HostView.cs:317)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize, System.Action onGUIHandler, System.Boolean canAffectFocus) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/IMGUIContainer.cs:403)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&) (at /Users/bokken/build/output/unity/unity/Modules/IMGUI/GUIUtility.cs:219)

Reproducible with versions: 6000.0.7f1

Not reproducible with versions: 6000.0.6f1

Tested on (OS): Mac (Intel)