I'm able to reproduce using the latest emacs-26 branch (I don't have the
point releases checked out at the moment), both lucid and gtk3 builds.
It doesn't reproduce using master, there is a Lisp error (Symbol's
definition is void: new-frame) that interferes.

It actually crashes as soon as I close the *GNU Emacs* frame, no chance
to type M-x.  I modified the .emacs to this: