https://forum.kicad.info/t/20jul18-v5-0-0-crash-in-3d-viewer/11617/10
The Error Window gets moved behind the KiCad Window. Minimize the KiCad Window and click “Cancel” on the Error Window; only needs to be done once per session from what I have observed.
ON EDIT: @davidsrsb ONLY the “Cancel” button ‘works’ on Windoze 7. You might want to alter your bug report with this information.