Fix NullPointerException on IDE startup in NotificationPopup

[4RH1T3CT0R7]

• Fix IDE fails to start: 'Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException' #12036 — IDE crashes on startup with NullPointerException at NotificationPopup.updateLocation() when Base.getActiveEditor() returns null before any
  editor window receives focus
• Add null guards in ContributionsSelfCheck and NewBoardListener before passing editor reference to NotificationPopup
• Add defensive null check in NotificationPopup.updateLocation() and constructor as defense-in-depth

Test plan

• Verify IDE starts without crash when board/library updates are pending
• Verify update notifications still appear after editor gains focus
• Verify notification popup follows editor window on move/resize

[CLAassistant]

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