[Excel import] Crash when saving a project with text data in the spreadsheet

• import the Excel file Financial_Sample.xlsx
• try to save the project and observe the crash

#0  QVector<QString>::destruct (this=<optimized out>, to=<optimized out>, from=0x1ae2f00) at /usr/include/qt5/QtCore/qvector.h:366
#1  QVector<QString>::freeData (x=0x1ae2ee0, this=<optimized out>) at /usr/include/qt5/QtCore/qvector.h:578
#2  0x00000000005431d9 in QVector<QString>::~QVector (this=0x1af3060, __in_chrg=<optimized out>) at /usr/include/qt5/QtCore/qvector.h:73
#3  ColumnClearCmd::~ColumnClearCmd (this=0x1e010b0, __in_chrg=<optimized out>) at /home/alex/Projekte/labplot/src/backend/core/column/columncommands.cpp:606
#4  0x0000000000543301 in ColumnClearCmd::~ColumnClearCmd (this=0x1e010b0, __in_chrg=<optimized out>) at /home/alex/Projekte/labplot/src/backend/core/column/columncommands.cpp:615
#5  0x00007ffff3fb42ce in QUndoStack::clear() () from /usr/lib64/libQt5Widgets.so.5
#6  0x00000000004c1756 in MainWin::save (this=this@entry=0x1083cf0, fileName=...) at /home/alex/Projekte/labplot/src/kdefrontend/MainWin.cpp:1736
#7  0x00000000004c1e6b in MainWin::saveProjectAs (this=this@entry=0x1083cf0) at /home/alex/Projekte/labplot/src/kdefrontend/MainWin.cpp:1687
#8  0x00000000004c20b1 in MainWin::saveProject (this=0x1083cf0) at /home/alex/Projekte/labplot/src/kdefrontend/MainWin.cpp:1655