Steps to reproduce:

There are actually steps before this – I was actually trying to delete some proxies, but to delete them, I first had to delete host groups, and to delete host groups, I had to make sure that each host had a host group, which means this path was much more frustrating than the "simple" frustration listed here.

1. Create a maintenance window (with a host group which in turn has a host)
2. Let the window expire (wait years!)
3. Retire the hosts
4. Decide you want to delete the host group (because you're decommissioning the project
5. Get an incomprehensible message
6. Try to find maintenance windows in general (this isn't fun if you don't regularly create maintenance windows, and most people don't, I haven't in years!)
7. See three maintenance windows and check each one trying to figure out what the heck is going on
8. Get annoyed and wonder if there's some other menu item for these objects (I searched each menu item)

Result:

Expected:

1. Ideally it should be possible to delete a host group even if there are expired maintenance windows that would be orphaned by this action
2. If an error is going to be annoying, it should provide a fast path to resolving it – this is especially important for things that are likely going to be in a series of "bridge out" "but I need to cross the gap" failure cases, and especially if the thing that users are likely to be looking for will probably be invisible. Here, a link to the specific object to be tweaked would be ideal. But, at least a note saying "By the way, the item you're looking for is expired, so you'll need to visit Configuration>Maintenance, select State: Expired (or Any) and then click Apply if you want to see it."