The pluginWidget of a VstPlugin (i.e. the VST editor) might be destroyed
outside VstPlugin's control resulting in a dangling pointer. This leads
to crashes for example when changing the model of an EffectView where a
VST plugin is attached, i.e. when re-opening a InstrumentTrackWindow of
an InstrumentTrack with a VST effect inside.
This can be fixed easily by using QPointer.
Closes#3045102.
(cherry picked from commit f4b84a6058)
48891a7de3