Try fix crash on get tabScoreView (from CrashReporter)

[igorkorsukov]

Resolves: https://sentry.musescore.org/musescore/editor/issues/9171 (from CrashReporter)
evens: 508
dump:

Fatal Error: EXCEPTION_ACCESS_VIOLATION_READ
MuseScore3 0x000140363167 Ms::ScoreTab::tabScoreView(int) c:\musescore\mscore\scoretab.h:54
Qt5Core 0x7ffaacc93180 <unknown>
Qt5Core 0x7ffaac953993 <unknown>
MuseScore3 0x000140363602 Ms::ScoreTab::viewSplitter(int) c:\musescore\mscore\scoretab.cpp:107
MuseScore3 0x0001403635b9 Ms::ScoreTab::view(int) c:\musescore\mscore\scoretab.cpp:95
MuseScore3 0x000140362069 Ms::ScoreTab::initScoreView(int,double,Ms::MagIdx,double,double) c:\musescore\mscore\scoretab.cpp:464
Qt5Core 0x7ffaac7b5287 <unknown>
MuseScore3 0x00014028496a Ms::MuseScore::restoreSession(bool) c:\musescore\mscore\musescore.cpp:5216

The single-line methods in the .h files do not motivate us to do state checks and do not allow us to see exactly which action failed.
Rewrote the code, added assertions.

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• I made sure the commit message title starts with "fix #424242:" if there is a related issue

[Harmoniker1]

I think these can be omitted...