Unity PropertyDrawer for selecting animation state names of AnimationControllers in the inspector

AnimationStateNameSelection.cs

// Place file in your main project, outside of an Editor folder
using UnityEngine;

namespace FMPUtils.Types
{
    [System.Serializable]
    public class AnimationStateNameSelection
    {
        public Animator animator;
        public string animationName;
    }
}

AnimationStateNameSelectionDrawer.cs

// Place file in your project, inside of an "Editor" folder
using FMPUtils.Types;
using System.Collections.Generic;
using UnityEditor;
using UnityEditor.Animations;
using UnityEngine;

namespace FMPUtils.Editor
{
    [CustomPropertyDrawer(typeof(AnimationStateNameSelection))]
    public class AnimationStateNameSelectionDrawer : PropertyDrawer
    {
        public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
        {
            return EditorGUIUtility.singleLineHeight * 2f;
        }

        public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
        {
            EditorGUI.BeginProperty(position, GUIContent.none, property);
            SerializedProperty animatorProp = property.FindPropertyRelative(nameof(AnimationStateNameSelection.animator));
            SerializedProperty animationNameProp = property.FindPropertyRelative(nameof(AnimationStateNameSelection.animationName));

            position = EditorGUI.PrefixLabel(position, GUIUtility.GetControlID(FocusType.Passive), label);
            Rect line1Rect = position; // Assignment copy since this is a struct
            line1Rect.height = EditorGUIUtility.singleLineHeight;
            Rect line1PrefixRect = line1Rect;
            float animatorPrefixLabelWidth = 80f;
            line1PrefixRect.width = animatorPrefixLabelWidth;
            Rect line1ValueRect = line1Rect;
            line1ValueRect.x = line1PrefixRect.x + line1PrefixRect.width;
            line1ValueRect.width = line1Rect.width - line1PrefixRect.width;

            EditorGUI.LabelField(line1PrefixRect, "Animator");
            animatorProp.objectReferenceValue = EditorGUI.ObjectField(line1ValueRect, animatorProp.objectReferenceValue, typeof(Animator), true);
			if (animatorProp.objectReferenceValue != null)
			{
                // Adjust the rect to draw contents one line below: 
                Rect line2Rect = position; 
                line2Rect.height = EditorGUIUtility.singleLineHeight;
                line2Rect.y = position.y + EditorGUIUtility.singleLineHeight;

                List<string> stateNames = new List<string>();
                string prevAnimationName = animationNameProp.stringValue;
                Animator animator = animatorProp.objectReferenceValue as Animator;
                AnimatorController animatorController = animator.runtimeAnimatorController as AnimatorController;

                // TODO: since there can be multiple animation layers it would be good to offer an additional dropdown for the layer selection 
                // before selecting the animation state name. 
                // States of substate machines seem to be included too but perhaps explore a way to better display/visualize it
                AnimatorControllerLayer[] layers = animatorController.layers;
                foreach (AnimatorControllerLayer layer in layers)
                {
                    foreach (ChildAnimatorState state in layer.stateMachine.states)
                    {
                        string stateName = state.state.name;
                        if (!stateNames.Contains(stateName))
                            stateNames.Add(stateName);
                    }
                }
                if (stateNames.Count == 0)
                {
                    EditorGUI.LabelField(line2Rect, "No states in AnimationController found");
                }
                else
                {
                    int selectedIndex = stateNames.IndexOf(prevAnimationName);
                    if (selectedIndex == -1)
                        selectedIndex = 0;

                    selectedIndex = EditorGUI.Popup(line2Rect, "State Name", selectedIndex, stateNames.ToArray());

                    if (selectedIndex >= 0)
                    {
                        animationNameProp.stringValue = stateNames[selectedIndex];
                    }
                    else
                    {
                        animationNameProp.stringValue = string.Empty;
                    }
                }
			}
			EditorGUI.EndProperty();
		}
    }
}