To the plugin author, this is easy to fix and needs to be done in the next version. It occurs when WP_DEBUG is enabled:
The fix is simple this case. The warning is caused by you checking the value of an array item inside $_POST without first checking that the key exists. PHP is very picky when you give it the chance, and demands that you never access array/object properties that odn't exist. When you want to use part of $_POST, like the 'aiosp_enabled' key that sets off the warning described above, you need to run isset() first. So instead of
if ($_POST['aiosp_enabled'] == null)
You should instead have something like
$aiosp_enabled = false;
$aiosp_enabled = $_POST['aiosp_enabled'];
if ($aiosp_enabled == null)
More fundamentally, you should enable WP_DEBUG while you are developing your plugins , this will let you know when your code is causing warnings and make it easy to fix them as you go. It is important that all plugins be WP_DEBUG-compatible because otherwise developers who run it to test their own plugins are forced to disable yours while doing so, which means they effectively haven't tested compatibilty with your plugin during the process.
Thanks in advance for fixing this. In this case it will be very simple to solve the problem, though once you enable WP_DEBUG you will probably find more simple issues to fix in your plugin.