fix: null reference exception when Container.Parent is null

[maxle5]

Exception:

01/16/2024 7:13:59 AM
System.NullReferenceException: Object reference not set to an instance of an object.
   at GlazeWM.Domain.Containers.Container.get_Siblings() in E:\GlazeWM\GlazeWM.Domain\Containers\Container.cs:line 46
   at GlazeWM.Domain.Containers.CommandHandlers.DetachContainerHandler.Handle(DetachContainerCommand command) in E:\GlazeWM\GlazeWM.Domain\Containers\CommandHandlers\DetachContainerHandler.cs:line 32
   at GlazeWM.Infrastructure.Bussing.Bus.Invoke[T](T command) in E:\GlazeWM\GlazeWM.Infrastructure\Bussing\Bus.cs:line 48
   at GlazeWM.Domain.Monitors.CommandHandlers.RemoveMonitorHandler.Handle(RemoveMonitorCommand command) in E:\GlazeWM\GlazeWM.Domain\Monitors\CommandHandlers\RemoveMonitorHandler.cs:line 47
   at GlazeWM.Infrastructure.Bussing.Bus.Invoke[T](T command) in E:\GlazeWM\GlazeWM.Infrastructure\Bussing\Bus.cs:line 48
   at GlazeWM.Domain.Monitors.CommandHandlers.RefreshMonitorStateHandler.Handle(RefreshMonitorStateCommand command) in E:\GlazeWM\GlazeWM.Domain\Monitors\CommandHandlers\RefreshMonitorStateHandler.cs:line 63
   at GlazeWM.Infrastructure.Bussing.Bus.Invoke[T](T command) in E:\GlazeWM\GlazeWM.Infrastructure\Bussing\Bus.cs:line 48
   at GlazeWM.Domain.Monitors.EventHandlers.DisplaySettingsChangedHandler.Handle(DisplaySettingsChangedEvent event) in E:\GlazeWM\GlazeWM.Domain\Monitors\EventHandlers\DisplaySettingsChangedHandler.cs:line 18
   at GlazeWM.Infrastructure.Bussing.Bus.Emit[T](T event) in E:\GlazeWM\GlazeWM.Infrastructure\Bussing\Bus.cs:line 87
   at GlazeWM.Infrastructure.Bussing.Bus.<>c__DisplayClass8_0`1.<EmitAsync>b__0() in E:\GlazeWM\GlazeWM.Infrastructure\Bussing\Bus.cs:line 101

Background:

• This occurs every time I wake my computer from sleep after having it previously connected to a monitor (e.g. laptop plugged into a monitor at work but not connected to a monitor when I get home).
• Seems to be caused by a recently added method Container.RemoveChild where the Parent property is set to null.

Only thing I'm not 100% sure about is whether or not I should have set the "fallbacks" for Index properties to -1 or to 0? I was worried setting to -1 would cause an index out of range exceptions somewhere...

[maxle5]

noticing some other issues due to this solution, closing until I can come back to this