TreeInstantiater

using System.Collections.Generic;
using UnityEditor;
using UnityEngine;
using EditorCallback = UnityEditor.EditorApplication.CallbackFunction;

[CustomEditor(typeof(Test))]
class Instantiater : Editor {

    private Transform parent {
        get {
            var find = GameObject.Find("Trees");
            if(find)
                return find.transform;
            return new GameObject("Trees").transform;
        }
    }

    private List<GameObject> prefabs = new List<GameObject>();
    private Vector2 scroll;

    public override void OnInspectorGUI() {
        scroll = EditorGUILayout.BeginScrollView(scroll, false, false);

        //parent = (Transform)EditorGUILayout.ObjectField("Parent transform", parent, typeof(Transform), false);

        ListSources();
        RemoveInvalidFiles();

        EditorGUILayout.EndScrollView();
    }

    private void OnSceneGUI() {
        if(prefabs.Count > 0) {
            HandleUtility.AddDefaultControl(GUIUtility.GetControlID(FocusType.Passive));
            var evt = Event.current;
            var cam = Camera.current;
            var mousePos = evt.mousePosition;
            mousePos.y = Mathf.Abs(mousePos.y - cam.pixelHeight);
            var ray = cam.ScreenPointToRay(mousePos);
            var hit = new RaycastHit();
            if(Physics.Raycast(ray, out hit)) {
                if(evt.type == EventType.MouseDown) {
                    var go = PrefabUtility.InstantiatePrefab(randomPrefab) as GameObject;
                    Undo.RegisterCreatedObjectUndo(go, go.name + " Creation");
                    go.transform.up = hit.normal;
                    go.transform.rotation = randomRot;
                    go.transform.position = hit.point;
                    go.transform.parent = parent;
                }
            }
            HandleUtility.Repaint();
        }
    }

    private Quaternion randomRot {
        get {
            return Quaternion.Euler(Vector3.up * Random.value * 360f);
        }
    }

    private GameObject randomPrefab {
        get {
            return prefabs[Random.Range(0, prefabs.Count)];
        }
    }

    private GameObject PrefabField(GameObject prefab) {
        var rect = EditorGUILayout.GetControlRect();
        var buttonRect = rect;
        buttonRect.xMin = rect.xMax - 15f;

        if(GUI.Button(buttonRect, "X", "ObjectFieldThumbOverlay"))
            return null;

        return PrefabField(prefab, rect);
    }

    private GameObject PrefabField(GameObject prefab, Rect rect) {
        rect.xMax -= 15f;
        return (GameObject)EditorGUI.ObjectField(rect, prefab, typeof(GameObject), false);
    }

    private GameObject[] DropAreaGUI() {
        var evt = Event.current;
        var dropArea = EditorGUILayout.GetControlRect();

        switch(evt.type) {
            case EventType.DragUpdated:
            case EventType.DragPerform:
            if(!dropArea.Contains(evt.mousePosition))
                return new GameObject[0];

            DragAndDrop.visualMode = DragAndDropVisualMode.Copy;

            if(evt.type == EventType.DragPerform) {
                DragAndDrop.AcceptDrag();

                var references = DragAndDrop.objectReferences;
                var list = new GameObject[references.Length];

                for(int i = 0; i < references.Length; i++)
                    list[i] = (GameObject)references[i];

                return list;
            }
            break;
        }

        var selectedFile = PrefabField(null, dropArea);
        if(selectedFile != null)
            return new GameObject[] { selectedFile };
        return new GameObject[0];
    }

    private void RemoveInvalidFiles() {
        for(int i = 0; i < prefabs.Count; i++)
            if(prefabs[i] == null)
                prefabs.RemoveAt(i);
    }

    private void ListSources() {
        EditorGUILayout.Space();
        EditorGUILayout.LabelField("Tree sources");

        for(int i = 0; i < prefabs.Count; i++)
            prefabs[i] = PrefabField(prefabs[i]);

        foreach(var file in DropAreaGUI())
            if(!prefabs.Contains(file))
                prefabs.Add(file);
    }
}