Fix #305106: Crash when applying preferences with no scores open

[Spire42]

Resolves: #305106

Added a missing nullptr check for mscore->currentScoreView() inside PreferenceDialog::apply(), fixing a crash that occurred if the user attempted to apply preferences when there were no scores open.

Also changed ScoreView::panSettings() to return a reference instead of a pointer, eliminating the need for an unnecessary nullptr check.

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