fix: error occurs if change model without initing, the model can't be sel… (#108)

When change model without initing, error occurs:
```shell
Uncaught TypeError: Cannot read properties of undefined (reading 'getElementsByClassName')
    at LLMChatInstance.resetChat (llm_chat.js:693:35)
    at LLMChatInstance.reboot (llm_chat.js:502:10)
    at HTMLSelectElement.onChange (llm_chat.js:785:25)
```

<img width="1360" alt="截屏2023-05-22 23 04 58"
src="https://github.com/mlc-ai/web-llm/assets/53991079/08ccac43-8517-491a-a99a-35e05198eeb6">

so the model can't be set correctly. check uichat element if occurs can
fix it.

Author: Ryan-yang125

web/llm_chat.js

@@ -687,6 +687,7 @@ class LLMChatInstance {
 
   resetChat() {
     if (this.requestInProgress) return;
+    if (!this.uiChat) return;
     const clearTags = ["left", "right"];
     for (const tag of clearTags) {
       const matches = this.uiChat.getElementsByClassName(`msg ${tag}-msg`);