fix: safely handle null pointers in thread API message processing

Added Optional chain for safe content extraction from OpenAI thread messages.
This prevents NullPointerException when message content, text, or value is null.

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

Co-Authored-By: Claude <[email protected]>

Author: Jiakaic

api/src/main/java/com/ke/bella/workflow/api/DifyController.java

@@ -1,5 +1,6 @@
 package com.ke.bella.workflow.api;
 
+import java.time.LocalDateTime;
 import java.time.ZoneId;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -18,13 +19,11 @@
 
 import javax.servlet.http.HttpServletRequest;
 
-import java.time.LocalDateTime;
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.format.annotation.DateTimeFormat;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.StringUtils;
-import org.springframework.format.annotation.DateTimeFormat;
-import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.PathVariable;
 import org.springframework.web.bind.annotation.PostMapping;
@@ -79,7 +78,9 @@
 import com.ke.bella.workflow.utils.JsonUtils;
 import com.ke.bella.workflow.utils.OpenAiUtils;
 import com.theokanning.openai.assistants.message.Message;
+import com.theokanning.openai.assistants.message.MessageContent;
 import com.theokanning.openai.assistants.message.MessageListSearchParameters;
+import com.theokanning.openai.assistants.message.content.Text;
 import com.theokanning.openai.service.OpenAiService;
 
 import lombok.AllArgsConstructor;
@@ -650,12 +651,28 @@ public Page<DifyChatFlowRun> pageChatFlowRuns(@PathVariable String workflowId,
                 }
 
                 for (List<Message> groupedMessage : groupedMessages) {
+                    String query = Optional.ofNullable(groupedMessage.get(0).getContent())
+                            .filter(content -> !content.isEmpty())
+                            .map(content -> content.get(0))
+                            .map(MessageContent::getText)
+                            .map(Text::getValue)
+                            .orElse("");
+
+                    String answer = groupedMessage.subList(1, groupedMessage.size()).stream()
+                            .map(e -> Optional.ofNullable(e.getContent())
+                                    .filter(content -> !content.isEmpty())
+                                    .map(content -> content.get(0))
+                                    .map(MessageContent::getText)
+                                    .map(Text::getValue)
+                                    .orElse(""))
+                            .filter(Objects::nonNull)
+                            .collect(Collectors.joining());
+
                     DifyChatFlowRun run = DifyChatFlowRun.builder()
                             .id(groupedMessage.get(0).getId())
                             .conversation_id(groupedMessage.get(0).getThreadId())
-                            .query(groupedMessage.get(0).getContent().get(0).getText().getValue())
-                            .answer(groupedMessage.subList(1, groupedMessage.size()).stream().map(e -> e.getContent().get(0).getText().getValue())
-                                    .collect(Collectors.joining()))
+                            .query(query)
+                            .answer(answer)
                             .created_at((long) groupedMessage.get(0).getCreatedAt())
                             .workflow_run_id(Optional.ofNullable(groupedMessage.get(0).getMetadata()).map(e -> e.get("workflowRunId")).orElse(null))
                             .build();