bug fix

APP_Framework/Applications/main.c

@@ -28,7 +28,7 @@ void TestFileSystem (){
         ret = write(fd_t, w_buf, strlen(w_buf));
         if(ret < 0){
         printf("fd = %d write hello world failed.\n",fd_t); 
-        return NULL;
+            return;
         }
         close(fd_t);