Thank you for your answers. I don't have a dark theme. I tried the
https://community.atlassian.com/t5/H...w/qaq-p/277535 solution but it didn't work. I search on internet "QT app open to white screen", I found things but the solutions were not understandable for me. Then I tried to install earlier versions of Sigil. I started with Sigil-0.8.0-Windows-x64 and it was working perfectly. All versions are working up to Sigil-0.9.16-Windows-x64. From Sigil-0.9.17-Windows-x64 up to Sigil-1.2.0-Windows-x64, all versions open with a white window.