Cleanup logic of PDT interpreter.

Assets/Cryville/Crtr/SkinInterpreter.cs

@@ -85,7 +85,7 @@ namespace Cryville.Crtr {
 					case '{':
 						return new SkinSelectors(s, a);
 					case '}':
-						return null;
+						throw new FormatException("Invalid token");
 					case '*':
 						GetChar();
 						compKeyFlag = true;