fix: patch view username in case of admin upload error

app.py

@@ -18,7 +18,7 @@ def update_data(excel_file):
         
 # Function to display a list of usernames
 def list_usernames():        
-    if type(sheets) == dict:
+    if (type(sheets) == dict and "employee_authentication" in sheets.keys()):
         data = sheets["employee_authentication"]
         usernames = data['ชื่อ-สกุลพนักงาน'].unique()
         # Display usernames in an HTML table for easy copying