Steps to reproduce the issue

Go to the menu items page:
joomla.example.com/administrator/index.php?option=com_menus&view=items
Choose a menu from the - Select menu - search tools dropdown.
Then go and edit the chosen menu:
joomla.example.com/administrator/index.php?option=com_menus&view=menu&layout=edit&id=<id of the menu type chosen in the previous page's search filter>
Change the menu's Menu Type.
Now go back to the first page. It crashes with a 500 Joomla error and there is no way to view the menu items anymore unless (I guess) logging out and logging back in or resetting the server session or reverting the menu type of the chosen menu to its old value.

Expected result

The joomla.example.com/administrator/index.php?option=com_menus&view=items page should work with the updated menu type in the filter or at least the filter should get reset.

Actual result

The joomla.example.com/administrator/index.php?option=com_menus&view=items page becomes unusable.

System information (as much as possible)

systeminfo-2017-10-03T17-39-37+03-00.txt

Additional comments

Worst case scenario the strict check for existing menu type should be removed from:
administrator/components/com_menus/models/items.php:150 (removing the second parameter from the call to MenusModelItems::getMenu() thus leaving the default value of false and preventing the raising of a fatal error)
...and reset the menu type filter user, app and view state by adding a fallback case in:
administrator/components/com_menus/models/items.php:159

else { // WE ALSO ADD A CASE WHICH RESETS THE FILTERS TO THEIR DEFAULT VALUES
	$app->input->set('limitstart', 0);
	$app->input->set('menutype', '');
	$app->setUserState($this->context . '.menutype', '');
	$this->setState('menutypetitle', '');
	$this->setState('menutypeid', '');
}

A better approach would be to add awareness for the user state to:
administrator/components/com_menus/models/menu.php:194 MenusModelMenu::save() method and update the user state with the new menu type value.