[PHP 8.2] fix undefined property in CRM_Event_Form_SelfSvcUpdate

[jofranz]

Overview

Playing around learning if this is enough to fix a PHP deprecation

Before

Test failed:
Creation of dynamic property CRM_Event_Form_SelfSvcUpdate::$_userContext is deprecated

After

Test doesn't fail anymore?

[civibot]

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[eileenmcnaughton]

Just doing a quick grep & it looks like that property might be set but never used?

[eileenmcnaughton]

Hmm this is annoying - I actually did do a cleanup on this form at some point but I can't find the PR - there are a bunch of undeclared properties in this one - I think #27770 is the gist of the previous work I did...

FYI - I think there are lots of easy wins on the php8.2 fails on the report tests

[eileenmcnaughton]

I'm gonna merge this - even though I think the property / setting of it can be removed - the ultimate goal of this PR was to see the impact of it on tests (while removing one notice) so I want to facilitate that