Going into Tools|NetBeans Platform Manager, the default platform will be the selected item. The Modules tab has a table
or list where all modules will be listed from the platform. Here it will say "Please wait..." or something like that
when waiting on modules to be enumerated. Sometimes this takes *much* longer than others. If you click on the "Add
Platform..." button while this is happening the IDE will hang and there is no way to keep working. Actually the only way
out is to kill the process with the OS. It seems when that background task is trying to locate those modules on disk
that should not have any other blocking consequences and that adding another platform should even be possible.