To reproduce:

Part I: Building

1) Open attached project
2) In top menu, select Build Tools -> Standalone -> (Re)create Standalone EXE -> Debug x64
3) The build will appear in /Binaries/ folder as "Debug" folder (the same place where Source folder is)
4) SessionRelay folder doesn't need to be rebuilt but will be used for reproducing

Part II: Testing

5) Open CMD/Powershell/Terminal
6) Navigate to the Debug/x64/SpritesheetGenerator.Standalone folder where you built the exe
7) Launch the exe through it, for example, 'SpritesheetGenerator.Standalone.exe -batchMode'. Note that no window will pop up because it's headless, but you should see the process running in task manager.
8) Navigate to SpritesheetGenerator.SessionRelay folder
9) Run SessionRelay.exe. You should see a message saying it has connected to SpritesheetGenerator.
10) Press '1' - "Send a small sprite sheet generation command (2 sprites)"
11) There should be messages saying commands are being sent to SpritesheetGenerator. Wait 10 seconds.
12) Spritesheet will be written out as PNGs in the [working directory]\spritesheets directory.
13) Open one of the sprite sheets. Notice how there are no avatars visible.

Reproduced with 5.6.0f3, 5.6.1f1, 2017.2.0b4

Not reproduced if built .exe file is not run in batch mode