Fix app to handle missing API key gracefully

- Implemented lazy initialization of conversation chain
- Added helpful error messages when API key is not set
- App now starts successfully even without API key configured
- Shows warning in UI when API key is missing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app.py

@@ -20,22 +20,22 @@ def get_session_history(session_id: str):
 def create_chain():
     groq_api_key = os.getenv("GROQ_API_KEY")
     if not groq_api_key:
-        raise ValueError("GROQ_API_KEY environment variable is not set")
-    
+        return None
+
     llm = ChatGroq(
         model="openai/gpt-oss-20b",
         temperature=0.7,
         max_retries=2,
     )
-    
+
     prompt = ChatPromptTemplate.from_messages([
         ("system", "You are a helpful AI assistant. Be concise and helpful."),
         MessagesPlaceholder(variable_name="history"),
         ("human", "{input}"),
     ])
-    
+
     chain = prompt | llm
-    
+
     return RunnableWithMessageHistory(
         chain,
         get_session_history,
@@ -43,9 +43,26 @@ def create_chain():
         history_messages_key="history",
     )
 
-conversation_chain = create_chain()
+# Lazy initialization - chain will be created on first use
+conversation_chain = None
 
 def chat(message, history):
+    global conversation_chain
+
+    # Check if API key is set
+    if not os.getenv("GROQ_API_KEY"):
+        error_msg = "⚠️ GROQ_API_KEY is not set. Please add it in the Space settings under 'Variables and secrets'."
+        history.append([message, error_msg])
+        return "", history
+
+    # Create chain on first use
+    if conversation_chain is None:
+        conversation_chain = create_chain()
+        if conversation_chain is None:
+            error_msg = "⚠️ Failed to initialize the chat model. Please check the API key."
+            history.append([message, error_msg])
+            return "", history
+
     try:
         response = conversation_chain.invoke(
             {"input": message},
@@ -61,7 +78,11 @@ def chat(message, history):
 with gr.Blocks(title="LangChain Groq Chat Demo") as demo:
     gr.Markdown("# LangChain + Groq Chat Demo")
     gr.Markdown("Chat with OpenAI's GPT-OSS-20B model using LangChain v1")
-    
+
+    # Show API key status
+    if not os.getenv("GROQ_API_KEY"):
+        gr.Markdown("⚠️ **Note:** GROQ_API_KEY is not configured. Please add it in Space settings to use this app.")
+
     chatbot = gr.Chatbot(height=400)
     msg = gr.Textbox(
         placeholder="Type your message here...",