Creates the portlet ID from context path and portlet name. The portlet ID
is constructed by concatinating the context path and the portlet name
using a dot ('.').
The method checks that the portlet name parameter does not have a dot. This check
is not done for the portlet ID.
Parameters: contextPath - the portlet context path. Parameters: portletName - the portlet name. throws: IllegalArgumentException - if the portletName has a dot throws: NullPointerException - if the portlet Name or context path is null.