Win 10 64 bit, happens regardless of which VST is loaded, but not in an empty VeSTige instance.

I get a segfault clicking that button on master. Is this the issue you're observing?
manageVestigeInstrumentView::manageVestigeInstrumentView tries to create a new subwindow with a null widget: https://github.com/LMMS/lmms/blob/ce5c74432e83f0eaa617cc26332f4209bc4efc7f/plugins/vestige/vestige.cpp#L910-L911
But MainWindow::addWindowedWidget tries to get the size hint for the given widget: https://github.com/LMMS/lmms/blob/ce5c74432e83f0eaa617cc26332f4209bc4efc7f/src/gui/MainWindow.cpp#L628
This line and the bug were introduced in c3b4d51. I guess we can just change the test to w && w->sizeHint().isValid().
I assume so.
@DomClark will you make a PR for this?
Could someone verify that it does not happen in stable too?
@Reflexe I've used this window a lot in 1.2 with no issues, but I just tested again to ensure it didn't regress in stable. Can't reproduce on stable 1.2.
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@Reflexe I've used this window a lot in 1.2 with no issues, but I just tested again to ensure it didn't regress in stable. Can't reproduce on stable 1.2.