修复bug

pages/classroom/classroom.js

@@ -67,7 +67,7 @@ Page({
   },
   send_message:function(event){
     let {message_text} = this.data;
-	if(message.trim()==""){return;}
+	if(message_text.trim()==""){return;}
     message = new Message({
       text: message_text,
       class: this.class,