Do you know why `invokeMethod` fails?
The qt documentation for that method states:
https://doc.qt.io/qt-5/qmetaobject.html#invokeMethod

> Returns false if there is no such member or the parameters did not match.

Could it be that `invokeMethod` is called before "slotFinalize" is installed?
If that is the case, is there a way to make sure that slot is installed early enough?

The tests for the buildbots sporadically fail for this BIST:

>>>>> processing /scratch/buildbot/workers/jwe-debian-x86_64-3/gcc-7-lto-debian/build/libinterp/corefcn/graphics.cc-tst
***** test
 hf = figure ("visible", "off");
 hax = axes ("parent", hf);
 unwind_protect
   hctx1 = uicontextmenu ("parent", hf);
   hctx2 = uicontextmenu ("parent", hf);
   set (hf, "uicontextmenu", hctx2);
   set (hax, "uicontextmenu", hctx2);
   assert (get (hf, "uicontextmenu"), hctx2);
   assert (get (hax, "uicontextmenu"), hctx2);
   assert (get (hf, "children"), [hctx2; hctx1; hax]);
   delete (hctx2);
   assert (get (hf, "uicontextmenu"), []);
   assert (get (hax, "uicontextmenu"), []);
   assert (get (hf, "children"), [hctx1; hax]);
   set (hf, "uicontextmenu", hctx1);
   assert (get (hf, "uicontextmenu"), hctx1);
   set (hf, "uicontextmenu", []);
   assert (get (hf, "uicontextmenu"), []);
   assert (get (hf, "children"), [hctx1; hax]);
 unwind_protect_cleanup
   close (hf);
 end_unwind_protect;
!!!!! test failed
ASSERT errors for:  assert (get (hf, "uicontextmenu"),[])
  Location  |  Observed  |  Expected  |  Reason
     .          O(1x1)       E(0x0)      Dimensions don't match

See e.g.:
http://buildbot.octave.org:8010/#/builders/24/builds/1235/steps/6/logs/stdio

This is probably a multi-threading issue that leads to the "uicontextmenu" property of the figure not being empty immediately after the corresponding "uicontextmenu" graphics object is deleted.

I also see this error randomly when running "make check" on my system.