The fetch command is asynchronous, the file is not guaranteed to be
available when command terminates.
I opened an issue at Mikrotik support (Ticket#2019041722004999),
their answer:
> You should perform a check in a loop.
> :delay until file exist
>
> That can happen also with any configuration not just files.
So add a function to wait for a file with given name.
I have not seen this with other configuration, though.
