user group two-column widget adds [object window] when nothing is selected

[jnweiger]

Seen with user_ldap 0.16.0 and owncloud server 10.9.0 RC2

• connect to an ldap server with more than 40 groups
• go to the users tab, select one group, e.g. 'Administrators' in the available groups column
  
• hit the [>] arrow, the selected entry moves to the selected group columns. OK

---

• without selecting anything, hit the [>] arrow again. The entry '[object Window]' appears. BAD
• it can be added multple times. BAD
  

Expected behavour:

• [object Window] is not a valid group. It should not appear in selected groups.
• Nothing should appear multiple times in the selected groups list.

[jnweiger]

Same in the groups tab.