Unity menu action to create an empty Editor for the selected MonoScript

CreateCustomEditorAction.cs

using UnityEditor;
using System.Text;
using System.IO;

public static class CreateCustomEditorAction
{
    [MenuItem("Assets/Create/Custom Editor from Script", true)]
    public static bool CreateCustomInspectorValidator()
    {
        return Selection.activeObject is MonoScript;
    }

    [MenuItem("Assets/Create/Custom Editor from Script")]
    public static void CreateCustomInspector()
    {
        // Creates a custom inspector for the given selected MonoScript.
        var selectedScript = Selection.activeObject as MonoScript;
        if (selectedScript == null)
            return;

        var scriptPath = AssetDatabase.GetAssetPath(selectedScript);
        if (string.IsNullOrEmpty(scriptPath))
            return;

        var directory = System.IO.Path.GetDirectoryName(scriptPath);
        var scriptName = System.IO.Path.GetFileNameWithoutExtension(scriptPath);

        var editorDirectory = System.IO.Path.Combine(directory, "Editor");

        if (!System.IO.Directory.Exists(editorDirectory))
        {
            System.IO.Directory.CreateDirectory(editorDirectory);
        }

        var editorScriptName = string.Format("{0}CustomEditor", scriptName);
        var editorScriptFile = string.Format("{0}.cs", editorScriptName);
        var editorScriptPath = System.IO.Path.Combine(editorDirectory, editorScriptFile);

        if (File.Exists(editorScriptPath))
            return;

        var contents = new StringBuilder();

        contents.Append("using UnityEngine;\n");
        contents.Append("using UnityEditor;\n");

        var scriptClass = selectedScript.GetClass();

        contents.Append("\n");
        contents.AppendFormat("[CustomEditor(typeof({0}))]\n", scriptClass.FullName);
        contents.AppendFormat("public class {0} : Editor\n", editorScriptName);
        contents.Append("{\n");
        contents.Append("\tpublic override void OnInspectorGUI()\n");
        contents.Append("\t{\n");
        contents.Append("\t\tDrawDefaultInspector();\n");
        contents.Append("\t}\n");
        contents.Append("\n");
        contents.Append("}\n\n");

        System.IO.File.WriteAllText(editorScriptPath, contents.ToString());

        AssetDatabase.ImportAsset(editorScriptPath);

        var editorScript = AssetDatabase.LoadAssetAtPath<MonoScript>(editorScriptPath);
        EditorGUIUtility.PingObject(editorScript);
        Selection.activeObject = editorScript;
    }
}