Change FontReference to reference type

FontReference is mutating itself by storing the cached font for later
reuse, but this is not working as intended because FontReference is
returned as a value type in UIControls so the cached font is actually
stored in a copy of the FontReference and not in the original property.

Source/Editor/Windows/Assets/FontWindow.cs

@@ -135,7 +135,7 @@ namespace FlaxEditor.Windows.Assets
         /// <inheritdoc />
         protected override void UnlinkItem()
         {
-            _textPreview.Font = new FontReference();
+            _textPreview.Font = null;
 
             base.UnlinkItem();
         }