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.