When trying to do any operations on the User Profile Service Application such as migrating users or groups (either PowerShell way or stsadm, you might get the “Value Cannot be Null. Parameter name: userProfileApplicationProxy” error like in the two screenshots right under:
The problem is that the current user doesn’t have the rights on the User Profile Service Application. To fix the error, go in Central Administration> Manage Service Applications > Highlight the User Profile Service Application. In the top ribbon, go in the Permissions and add your current account with Full Control. Afterwards, with the User Profile Service Application still selected, select the Administrators button and add the current user account.
Everything should work afterwards.