Fix file not found error for get_preview script (#381)

* Fix file not found error for get_preview script

backend/utils/github.py

@@ -48,8 +48,11 @@ def get_file(download_url: str, file: str, branch: str = 'HEAD') -> [dict, None]
     local_workspace = os.getenv("GITHUB_WORKSPACE")
     if local_workspace:
         # read files locally since github action already checked it out
-        with open(os.path.join(local_workspace, file)) as f:
-            return f.read()
+        if os.path.exists(os.path.join(local_workspace, file)):
+            with open(os.path.join(local_workspace, file)) as f:
+                return f.read()
+        else:
+            return None
 
     api_url = download_url.replace("https://github.com/",
                                    "https://raw.githubusercontent.com/")