Fix #337411: Crash with Qt 5.15 (on Linux / with Mu3.7)

[Jojo-Schmitz]

By adding if (!beatBaseList) return;. Also fix some QtCreator warnings reg. != and == as well as 2 QtCreator errors reg. Ids being enclosed in '

Resolves: https://musescore.org/en/node/337411 (maybe)
Resolves: #27 (certainly)

[Jojo-Schmitz]

Some of those ' to " changes are mandated by QtCreator


Therse two actually need removeing them rather than replacing them.
The others (the real changes from ' to ") seem to be purly cosmetical, and I can easily revert them it you don't like them (you mentioned the ' enclosed strings to be more performant in JavaScript on Discord?), so I'll do that now.
Changes from != and == to !== and ===, respectively, are to fix QtCreator warnings. e.g:

The real fix for the crash though seems that additional check and return, if (!beatBaseList) return;, IIRC (it is 3 years ago after all)
It certainly does seem to resolve #27.