Filesystem long initialisation can cause issues accessing the files modal early

The long load times of the microbit-fs library and the processing of hex files on page load means it could take several seconds for the filesystem to be ready.
If the files modal is opened before this, it will not show any files, be able to load any files and could throw some errors.
This can also be noticeable if the user drag&drops a file into the editor as soon as it is loaded.

首先跟踪 microbit-fs 的初始化过程，以及 issue 中描述的 files modal 和 drag-and-drop 入口点。重现页面加载后立即打开 modal 或拖放文件的操作；当文件系统仍在初始化时这些操作能够可靠运行，即表示完成。

由索引模型根据 Issue 内容生成。