You are right, the item field is required, but It souldn't.
Corrected since 1.3.3
Regenerate your project, or if you want to correct this in your component :
FRONT/com_mycomponent/views/myview/tmpl/mylayout.xml
CODE:
<!-- Add fields to the request variables for the layout. -->
<fields name="request">
<fieldset name="request"
addfieldpath="/administrator/components/com_mycomponent/models/fields">
<field name="cid" type="modal_itemitem"
label="MYCOMPONENT_MENU_SELECT_ITEM"
required="true"
description="MYCOMPONENT_MENU_SELECT_THE_ITEM_TO_LINK_ON_THIS_MENU"
/>
</fieldset>
</fields>
CHANGE :
TO :