Update app.py

app.py

@@ -6,8 +6,12 @@ client = OpenAI(api_key=os.getenv("OPENAI_API_KEY"))
 
 # ---- GPT-5 explanation backend ----
 def explain_text(selected_text):
-    if not selected_text.strip():
-        return "Highlight some text first 👆"
+    if selected_text is None:
+        return ""
+
+    selected_text = selected_text.strip()
+    if not selected_text:
+        return "Please select or enter some text first."
 
     prompt = f"""
 You are an expert machine learning instructor.