Description:
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If you define a password compare rule after a frozen element, you'll get an error like:
Fatal error: Call to a member function on a non-object in QuickForm.php on line 1563

After 2 hours of researching I found the error. In the method  "getValidationScript" is a loop, where the variable $element points to $this->_elements[$elementKey]. If the element is frozen, the loop will be skipped. In the next run of the loop the password compare rule will be handled. The variable $dependent will be true, and the object $element will be overwritten with an array(). But $element still points to $this->_elements[$elementKey] like i mentioned above and so $this->_elements[$elementKey] will also be overwritten.
This causes the error.

The solution is to add unset($element) before the lopp will be skipped.

Here is the correct Code:

Reproduce code:
---------------
// Line 1669 in Script HTML/Quickform.php
                    // No JavaScript validation for frozen elements
                    if (is_object($element) && $element->isFrozen()) {
                        unset($element);
                        continue 2;
                    } elseif (is_array($element)) {
                        foreach (array_keys($element) as $key) {
                            if ($element[$key]->isFrozen()) {
                                unset($element);
                                continue 3;
                            }
                        }
                    }