The "Find unreferenced assets" feature is broken for YYC games

[VladiStep]

Describe the bug

When you try to use the feature, it shows the NullReferenceException, because it tries to access data.Code, even if it's a YYC game.

Reproducing steps

1. Load a YYC game (or simply make the Data.Code null).
2. Open "Find unreferenced assets".
3. Select any type.
4. Click "Search".

Setup Details

1. UTMT version - latest (0.9.2.0)