You need to inherent SaintsField.Editor.SaintsEditor:

[CustomEditor(typeof(MyMonoBehavior))]
public class LabelTestEditor : SaintsField.Editor.SaintsEditor  // <-- Use this
{
    public override VisualElement CreateInspectorGUI()
    {
        VisualElement root = new VisualElement();

        root.Add(base.CreateInspectorGUI());  // fill the default fields

        // If you want to use IMGUI, put it inside IMGUIContainer
        root.Add(new IMGUIContainer(() =>
        {
            using (new EditorGUILayout.HorizontalScope())
            {
                GUILayout.TextField("IMGUI Text Field");
                GUILayout.Button("IMGUI Button");
            }
        }));

        return root;
    }
}