I get an error when running the game.
```
Creating OpenGL window...
debugger invoked on a TYPE-ERROR in thread
#<THREAD "main thread" RUNNING {10027FE5C3}>:
The value
NIL
is not of type
SB-SYS:SYSTEM-AREA-POINTER
Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.
restarts (invokable by number or by possibly-abbreviated name):
0: [ABORT] Exit debugger, returning to top level.
(LISPBUILDER-SDL-BASE:SURF-W NIL)
```