Everytime I start my MacBook Air there is a blank interface window
I have solved the problem by reinstalling latest OS. Follow these steps to do it :


Shut down your Mac first
Restart and keep cmd+R buttons pressed until Apple Logo shown
Choose from the menu reinstall OS (in my case : Reinstall Sierra)
Follow on screen instructions.


Reinstalling latest OS doesn't change or delete your current settings or installed softwares.