SetupArrayProps R: array prop 'null' is at index zero

The startup error SetupArrayProps R: array prop 'null' is at index zero is caused by trying to run a Source 2007 game/mod with the Source 2009 engine. This is not possible.

Solution

The only Source 2007 game left today is Source SDK Base 2007. To use it:

1. Change the SteamAppId value in gameinfo.txt to 218, then restart Steam.
2. Make sure that any paths to hl2.exe (such as the launch path for the Visual Studio debugger) are pointing to SteamApps/account_name/source sdk base 2007/hl2.exe.
3. If the SDK launcher lists any Source 2007 games, reset it for each engine version to prevent future confusion.
4. Use the SDK launcher's "Edit Game Configurations" tool to move your mod's SDK profile to Source 2007.

 Tip: To access content from a 2009 game while still using the 2007 engine, see mounting multiple games.