The issue only can be reproduced on ee-6.2.x. Since 7.0 changed the UI design when edit ruser, so the issue won't occur.
STEPS TO REPRODUCE:
1. Add a new user "test1"
2. Assign user "test1" to the "Liferay" site.
3. Save. User is assigned to the site as expected.
4. Remove user "test1" from the "Liferay" site.
5. Save. User is removed from the site as expected.
6. Stop portal.
7. Change the property "users.form.update.main" by adding the following to portal-ext.properties:
8. Restart portal.
9. Log into control panel.
>. Assign user "test1" to the "Liferay" site>. Save.
Check: User is assigned to the site as expected.
10. Remove user "test1" from the "Liferay" site.
Current behavior: User can not be removed from site and the following js error reported in browser: Uncaught TypeError: Util.getTop(...).Liferay.Util.Window.getById is not a function. The same issue also occur in organizations,sites,user-groups,roles section.
Expected behavior: user can be removed from site successfully without errors.