Fix NonType AttributeError in chameleon_screen_test

BUG=None
TEST=None. The fix is trivial.

Change-Id: I567b5569015444344bed196240b56d03b7818661
Reviewed-on: https://chromium-review.googlesource.com/237719
Tested-by: Hung-ying Tyan <tyanh@chromium.org>
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
Commit-Queue: Hung-ying Tyan <tyanh@chromium.org>
diff --git a/client/cros/chameleon/chameleon_screen_test.py b/client/cros/chameleon/chameleon_screen_test.py
index 711378e..e6e5d96 100644
--- a/client/cros/chameleon/chameleon_screen_test.py
+++ b/client/cros/chameleon/chameleon_screen_test.py
@@ -114,7 +114,8 @@
                 if error is None:
                     return error
                 elif retry_count == 0:
-                    error_list.append(error)
+                    if error_list is not None:
+                        error_list.append(error)
                     return error
                 else:
                     logging.info('Retry screen comparison again...')