backends/x11/standalone: add a nullptr check in the vblank handler
It can be called without a previous presentation in some cases
(cherry picked from commit 3bf97e87)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com
It can be called without a previous presentation in some cases
(cherry picked from commit 3bf97e87)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com