Returns the next component UUID for the specified component,
or null to generate the default UUID.
Default (if null is returned): UUID is generated by prefixing
a portion of the desktop's ID with a number starting from 0.
Notice that UUID cannot be empty and can only have alphanumeric characters or underscore
(i.e., a-z, A-Z, 0-9, and _).
This method is called when
Component#getUuidis called at the first time. It is usually when
the component is attached to a page, unless the application
invokes
Component#getUuid earlier.