This issue was identified using valgrind. Output:

==200828== Mismatched free() / delete / delete []
==200828==    at 0x4C2E668: free (in
/nix/store/270csszcn55xv52pjmvgr6cgy5grc8y5-valgrind-3.14.0/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==200828==    by 0xAFCBFD0: mlt_profile_close (in
/nix/store/l1dy6sf0sf6j7r7m45sg0xa8i8fxw4zq-mlt-6.13.0/lib/libmlt.so.6.13.0)
==200828==    by 0x7E52F8: GLWidget::~GLWidget() (glwidget.cpp:173)