Add support for displaying Multiple Values info to object reference pickers

#2984

Source/Editor/GUI/AssetPicker.cs

@@ -48,6 +48,11 @@ namespace FlaxEditor.GUI
         /// </summary>
         public bool CanEdit = true;
 
+        /// <summary>
+        /// Utility flag used to indicate that there are different values assigned to this reference editor and user should be informed about it.
+        /// </summary>
+        public bool DifferentValues;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="AssetPicker"/> class.
         /// </summary>
@@ -121,7 +126,13 @@ namespace FlaxEditor.GUI
             if (CanEdit)
                 Render2D.DrawSprite(style.ArrowDown, button1Rect, button1Rect.Contains(_mousePos) ? style.Foreground : style.ForegroundGrey);
 
-            if (Validator.SelectedItem != null)
+            if (DifferentValues)
+            {
+                // No element selected
+                Render2D.FillRectangle(iconRect, style.BackgroundNormal);
+                Render2D.DrawText(style.FontMedium, "Multiple\nValues", iconRect, style.Foreground, TextAlignment.Center, TextAlignment.Center, TextWrapping.NoWrap, 1.0f, Height / DefaultIconSize);
+            }
+            else if (Validator.SelectedItem != null)
             {
                 // Draw item preview
                 Validator.SelectedItem.DrawThumbnail(ref iconRect);