gtk: Do not assume parent impl of `GLArea.create_context` returns non-null pointer#2156

bilelmoussaoui merged 1 commit intogtk-rs:maingtk-rs/gtk4-rs:mainfrom

yvt:fix-gtk4-subclass-glarea-null-contextyvt/gtk4-rs:fix-gtk4-subclass-glarea-null-contextCopy head branch name to clipboard