[4.2] Fix issue "updateCheck is null"

[fancyFranci]

Pull Request for Issue #38513.

Summary of Changes

Going to the joomla updater can lead to a js error because an optional element can not be found. This PR checks the element before it is used.

Testing Instructions

Short: Follow the instructions in the linked issue.
Long:
Open the developer tools of your browser to see the console.
Go to System -> Global Configuration -> Joomla! Update.
Try out different channels.
Everytime you click "Save & Close" you are redirected to the update page.
There you can click buttons like "Reinstall Joomla! core files"

Actual result BEFORE applying this Pull Request

An js error saying "TypeError: updateCheck is null" appears in the browser console. Depending on your debug settings it could say "a is null" or something like this. Buttons are not working.

Expected result AFTER applying this Pull Request

There is no js error and buttons should work as always.

Documentation Changes Required

Nope.

[zero-24]

I have tested this item ✅ successfully on 65394e0

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38519.}

[richard67]

I have tested this item ✅ successfully on 65394e0

I did not get a JS error in my browser console without this PR. But I can confirm that without this PR, the button to reinstall the core files doesn't work, and with this PR it does work.

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38519.}

[richard67]

RTC

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38519.}

[richard67]

RTC !

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38519.}

[roland-d]

Thank you