Score causes crash on load, cross measure beam

See https://musescore.org/en/node/288788

Score created (last saved with) 3.0.5. Crashes there and in latest (post 3.1beta2) master too

Stack trace:
1 std::vector::operator[] stl_vector.h 816 0xe0ca80
2 Ms::Segment::element segment.h 116 0xdd30fb
3 Ms::breakCrossMeasureBeams layout.cpp 2405 0x87e805
4 Ms::Score::collectSystem layout.cpp 3482 0x883cf2
5 Ms::LayoutContext::collectPage layout.cpp 4153 0x88809c
6 Ms::LayoutContext::layout layout.cpp 4457 0x889ca4
7 Ms::Score::doLayoutRange layout.cpp 4443 0x889ba3
8 Ms::Score::update cmd.cpp 220 0x98b0a9
9 Ms::readScore file.cpp 2345 0x647ea5
10 Ms::MuseScore::readScore file.cpp 346 0x636440
11 Ms::MuseScore::openScore file.cpp 324 0x636305
12 Ms::MuseScore::loadFiles file.cpp 304 0x635ffa
13 Ms::MuseScore::cmd musescore.cpp 5927 0x4f57e8
14 Ms::MuseScore::cmd musescore.cpp 5735 0x4f49a3
15 Ms::MuseScore::qt_static_metacall moc_musescore.cpp 855 0x40a29a
16 ZN11QMetaObject8activateEP7QObjectiiPPv 0x68c81805
17 ZN12QActionGroup7hoveredEP7QAction 0x264b4ee5

With that crash avoided by checking for that nulptr in Ms::breakCrossMeasureBeams layout.cpp line 2405 it reports a corruption:

Measure 82, Staff 4 incomplete. Expected: 4/4; Found: 0/1

Ignoring that and the score loads. But it then crashes on (auto-) saving the score with an assertion failure:

Fatal: ASSERT: "first()" in file ...\MuseScore\libmscore\measure.cpp, line 1918 (...\MuseScore\libmscore\measure.cpp:1918, )