Fix segfault when hovering a Qt figure while it's being drawn (bug #45540) * Canvas.cc (Canvas::canvasMouseMoveEvent): ensure the QtHandles::Figure object is built from a figure object and is non null * Canvas.cc (Canvas::canvasMouseMoveEvent): add a FIXME note

%!function r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10");