virt-viewer won't close - "Waiting for display 1..."

I've installed spice-vdagentd in my NixOS client in QEMU/virt-manager and now everytime i poweroff the machine, the virt-viewer-window won't close. It shows the message "Waiting for display 1..." and i can only close it by killing the virt-viewer-process.

Can someone tell me why this is and how i might resolve it?