Thanks for the bug report.
The slash issues have been fixed and this will work correctly in the next version.
I'm a bit surprised that this is the first report of this problem.

It looks like LoadAsset<T>() at 156 OnlineMapsEditorUtils.cs needs a:

filename = filename.Replace('\\', '/');

Otherwise, on Mac, Maps gets stuck in an fatal exception loop where Maps tries to load assets for the helper window and LoadAsset() forms a path with mixed path chars (first half is '/' and second half is '\\' from what is passed in).  Worse, it keeps looping.   I suspect this affects your Mac sales.