Grey out the context menu item if no urdf is selected (#18)

Author: LaurieCheers-unity

Editor/MenuItems/UrdfImporterContextMenuItem.cs

@@ -7,7 +7,15 @@
 namespace RosSharp.Urdf.Editor
 {
     public static class UrdfImporterContextMenuItem
-    {
+    {
+        [MenuItem("Assets/Import Robot from URDF", true, 0)]
+        public static bool CreateUrdfObject_IsValid()
+        {
+            string assetPath = AssetDatabase.GetAssetPath(Selection.activeObject);
+
+            return (Path.GetExtension(assetPath)?.ToLower() == ".urdf");
+        }
+
         [MenuItem("Assets/Import Robot from URDF")]
         private static void CreateUrdfObject()
         {