Fix crash due to dangling pointer access in developer mode.

[daschuer]

This fixes a regression #13885 since #12094

[daschuer]

Done

[Swiftb0y]

LGTM. Thank you.

[JoergAtGithub]

@daschuer Thank you for jumping in! The fix LGTM and I just tested it on Win11 -> Works as expected!

[ronso0]

Thanks @daschuer for quick fix!
Can confirm it doesn't crash anymore.

However, I want to note that I saw the crash for other event types, too, eg. QChildEvent(ChildRemoved, ..).
So, presuming

• Add a debug message, which appears when event loop processing in Mixxx application takes very long #12094 is for diagnosis only
• we can't be sure it happens only for QEvent::DeferredDelete
• a crash can cause losing data from the current session
• users may run Mixxx with --developer in regular session to have MIDI Through for MIDI4Light

shall we tie to another developer option, eg. a certain log level?

[JoergAtGithub]

However, I want to note that I saw the crash for other event types, too, eg. QChildEvent(ChildRemoved, ..).

Than I see only two options:

1. Cache the object informantion before calling notify, which will affect the general performance in developer mode
2. Remove the printout of the target object information and just print the information about the event type

[ronso0]

3. implementing Display MIDI Through device in non-developer mode #8356 would "fix" it IMO since this eliminates the only reason I know to run Mixxx with --developer in regular sessions