Add font fallback

Note: All the `First()` in the code are temperary workarounds to make it work and require refractoring

Source/Editor/GUI/ComboBox.cs

@@ -273,7 +273,7 @@ namespace FlaxEditor.GUI
             MaximumItemsInViewCount = 20;
 
             var style = Style.Current;
-            Font = new FontReference(style.FontMedium);
+            Font = new FontReference(style.FontMedium.First());
             TextColor = style.Foreground;
             BackgroundColor = style.BackgroundNormal;
             BackgroundColorHighlighted = BackgroundColor;