Material upgrader now displays a dialog informing that at least one material must be selected when clicking on upgrade selection option.

ScriptableRenderPipeline/Core/CoreRP/Editor/MaterialUpgrader.cs

         public static void UpgradeSelection(List<MaterialUpgrader> upgraders, string progressBarName, UpgradeFlags flags = UpgradeFlags.None)
         {
             var selection = Selection.objects;
+
+            if (selection == null || selection.Length == 0)
+                if (EditorUtility.DisplayDialog("Material Upgrader", "You must select at least one material.", "Ok"))
+                    return;
+
+            Debug.Log(selection.Length);
             for (int i = 0; i < selection.Length; i++)
             {
                 if (UnityEditor.EditorUtility.DisplayCancelableProgressBar(progressBarName, string.Format("({0} of {1}) {2}", i, selection.Length, lastMaterialName), (float)i / (float)selection.Length))