Fix an analysis hint. (#56)

diff --git a/lib/command_runner.dart b/lib/command_runner.dart
index 5780fae..fa22c23 100644
--- a/lib/command_runner.dart
+++ b/lib/command_runner.dart
@@ -121,6 +121,7 @@
       }
 
       command.usageException(error.message);
+      return null;
     }
   }