I can confirm that Pcbnew freezes when opened from Kicad. It won't fully initialize UI and attempt to close the window would crash the app.
The core reason seems to be mismatched versions of the wxWidgets (wxgtk) and wxPython. The problem can be resolved by disabling WxPython scripting or ensuring versions match (which may be difficult to achieve long-term):
-DKICAD_SCRIPTING_WXPYTHON=OFF
Not sure how many people are using this part of scripting. KiCad 5 is a very nice improvement over version 4 and I recommend it to everyone! :)
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