Thank you for writing.
It seems that you have some HostWindows in the collection which you are attempting to cast to ToolWindows, thus the observed exception. Such host windows might be MDI children added to RadDock for example.
To overcome the error, you can iterate the collection by using DockWindow instead of HostWindow and then only if you find a ToolWindow, cast to it and call its Float method:
MyLocation.X = 50
MyLocation.Y = 50
MySize.Width = 1090
MySize.Height = 700
(ToolWindow, ToolWindow).Float(MyLocation, MySize)
I hope this helps.