windows: nil reference panic while switching remote desktop sessions

[AnyCPU]

Describe the bug

at some point of time it is possible that a Screens' slice is empty and a window needs to be updated.

it was discovered during my work on https://github.com/cogentcore/core/issues/1458

How to reproduce

1. run a Core app on Windows machine;
2. run a Windows App (Microsoft Remote Desktop) on macOS;
3. connect to the Windows machine.

Example code

Relevant output

Platform

Windows