Fix crash at start when rulers-follow-grid is enabled

When creating the editor, default modes are set, this toggles
mouse-modes, in `Editor::mouse_mode_chosen` which in turn calls
EditingContext::grid_type_chosen.

With rulers-follow-grid enabled this can call
`Editor::toggle_ruler_visibility`. Since `_session` is NULL
during editor's c-tor this caused a crash in
`Editor::store_ruler_visibility`.

gtk2_ardour/editor_actions.cc

@@ -777,6 +777,8 @@ Editor::toggle_ruler_visibility ()
 		return;
 	}
 
+	assert (_session);
+
 	update_ruler_visibility ();
 	store_ruler_visibility ();
 }