fix Qt6 deprecation warning in mapbar_track. (#703)

fix Qt6 deprecation warning in mapbar_track. (#703)

* fix Qt6 deprecation warning in mapbar_track.

"warning: 'fromUtf16' is deprecated: Use char16_t* overload."
However, it is recommended to use QString(const QChar *, int) or
QString(const QChar *) instead of
QString::fromUtf16(const ushort *unicode, int size = -1) and
QString::fromUtf16(const char16_t *str, int size = -1) anyway.

* fix Either the condition 'track==nullptr' is redundant or there is possible null pointer dereference.

as this isn't using a non-throwing allocation function memory allocation
errors will cause std::bad_alloc to be thrown anyway, we won't return
with track == nullptr.

* eliminate another non-functional memory check.