fix nonexistent upload error

pages/upload.py

@@ -44,21 +44,24 @@ def app():
                 data.to_csv('data.csv', index=False)
     
         if st.button("Load Data"):
-            st.markdown("### Confirm the data is correct.")
-            num_items = 0
-            form = st.form("data_check_form")
-            with open("data.csv", "r") as f:
-                reader = csv.reader(f, delimiter=",")
-                for i, line in enumerate(reader):
-                    if i == 0:
-                        pass
-                    else:
-                        num_items += 1
-                        form.markdown(f"### {i}")
-                        form.text_input("Word or Phrase", f"{line[0]}", key=f"word_{i}")
-                        form.text_input("Definition", f"{line[1]}", key=f"def_{i}")
-                        form.text_input("Example", f"{line[2]}", key=f"ex_{i}")
-                        form.text_input("Tags", f"{line[3]}", key=f"tag_{i}")
-            form.form_submit_button("Confirm", on_click=upload_callback, args=(num_items,))
+            if not os.path.exists("data.csv"):
+                st.warnining("Upload a file to load data.")
+            else:
+                st.markdown("### Confirm the data is correct.")
+                num_items = 0
+                form = st.form("data_check_form")
+                with open("data.csv", "r") as f:
+                    reader = csv.reader(f, delimiter=",")
+                    for i, line in enumerate(reader):
+                        if i == 0:
+                            pass
+                        else:
+                            num_items += 1
+                            form.markdown(f"### {i}")
+                            form.text_input("Word or Phrase", f"{line[0]}", key=f"word_{i}")
+                            form.text_input("Definition", f"{line[1]}", key=f"def_{i}")
+                            form.text_input("Example", f"{line[2]}", key=f"ex_{i}")
+                            form.text_input("Tags", f"{line[3]}", key=f"tag_{i}")
+                form.form_submit_button("Confirm", on_click=upload_callback, args=(num_items,))
     # st.text_input(f'{q[0] + 1}. {q[3]}', key=q[0], placeholder="Type answer here")
     # st.form_submit_button(label="Submit", on_click=form_callback, args=(questions,))