E_NOTICE thrown by element_children() on user/%uid/edit page

No. In that case $key[0] would be 't' and no notice would be thrown.

The problem comes from this

        // For the default theme, revert to an empty string so the user's theme updates when the site theme is changed.
        $info->key = $info->name == variable_get('theme_default', 'garland') ? '' : $info->name;

in system_theme_select_form().

And now that I look at that code I see an usability problem too. A user can't select a default theme to be his default theme. If the site's default theme changes his theme changes too, even if he actually selected a certain theme (which happen to be the default theme of the site at that time).

Now, do we solve the initial problem? I know drupal doesn't babysits broken code, but I don't know if using an array with an empty key qualifies as broken code.

For the usability problem, I think that theme selection form on user edit page should have an option "Use default theme" which should use 'use_default_theme' as $key to avoid the Notice message.

This patch treats the root of problem instead of just making it shut up.

Lets see if this one passes.

Because element_child() does an foreach $key => $value and inside that does a $key[0] which should get the first character of the string, but if there is no string in throws a Notice.

Why is element_child() called on that is above my expertise right now.

Edit: I opened #517662: Can't select site's default theme as user's theme for the usability problem.

Fix to the title :)