UI Toolkit (built in, no package) fails to build on 2021.2 Alpha with IL2CPP.

[Midiphony-panda]

Similar issue with Unity 2020.2.6, UITK preview 14, with IL2CPP built players, but only when using ThemeStyleSheets :

Serialization issue with ThemeStyleSheets (Unity 2020.2.6 - IL2CPP - UITK runtime preview 14)

Hello, I saw in another thread that ThemeStyleSheets will come in UITK preview 15 and for the 2021.2 release. We took a look on it with Unity 2020.2.6 + UITK preview 14 and had some surprises with built players. When using a ThemeStyleSheet in the PanelSettings, everything is fine in the editor and with Mono built players. However, there are some serialization issues with ThemeStyleSheets in built players when the scripting backend is set to IL2CPP (and only IL2CPP). We get the following…

Case 1325966