This is usually caused by another plugin that doesn’t use some of the WordPress filters properly and instead of returning array, returns null.
If you’re sure it started after importing the settings, try saving the TinyMCE Advanced’s options again and export the settings again.
Now I installed and activated TinyMCE Advanced on the empty site with WordPress 4.7.3. No other plugins are activated.
After importing TinyMCE Advanced’s settings I have got the same PHP Notice. I saved the TinyMCE Advanced’s options again, exported the settings again and then imported these saved options – and received the same message.
May be it does not influence on the work of the plugin and the site.
Lucks