Fix EditStaff dialog crash on startup

[CubikingChill]

The crash occurred because initStaff() was called in the constructor before the context injection system was fully initialized. After the MUSE_MULTICONTEXT_WIP removal, globalContext() returns null during construction, causing a segfault when notation() is called.

Fixed by moving initStaff() from the constructor to showEvent(), ensuring the context is ready before accessing it.

Resolves: #32743

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[igorkorsukov]

replaced by #32757