fix: index issues

forgot to commit on my pc, so doing it on laptop

src/ts/process/memory/hypav2.ts

@@ -160,14 +160,20 @@ function cleanInvalidChunks(
     data.mainChunks = data.mainChunks.filter((mainChunk) => {
         return isSubset(mainChunk.chatMemos, currentChatMemos);
     });
+
     // chunk filtering based on mainChunk's id
     const validMainChunkIds = new Set(data.mainChunks.map((mainChunk) => mainChunk.id));
     data.chunks = data.chunks.filter((chunk) =>
         validMainChunkIds.has(chunk.mainChunkID)
     );
-    data.lastMainChunkId = data.mainChunks[-1].id; // Quite literally the definition of lastMainChunkId. Didn't use .length, since middle chat context can be partially deleted.
-
+    // Update lastMainChunkId
+    if (data.mainChunks.length > 0) {
+        data.lastMainChunkId = data.mainChunks[data.mainChunks.length - 1].id;
+    } else {
+        data.lastMainChunkId = 0;
+    }
 }
+
 export async function regenerateSummary(
     chats: OpenAIChat[],
     data: HypaV2Data,